Many of us think "constipation" means stools that are not as soft as usual. This isn't quite true: stool consistency changes all the time, depending on what and when we eat and drink and when and how often we have bowel movements. Strictly speaking, "constipation" refers to infrequent or incomplete bowel movements. We also use the term to refer to stools that are hard or difficult to pass. (Bear in mind that soft but bulky stools, like you have after eating high-fiber foods, can also be hard to pass.)
When we eat, the food we eat is mixed with acids and some enzymes in the stomach (after we grind it up with our teeth), and the mixture -- which is a thin liquid -- passes into the small intestine where it is mixed with other enzymes and with bile (a liquid secreted by the liver, which contains chemicals that also help break down food and also contains waste products that the liver removes from your blood, including breakdown products of old red blood cells, some medicines that you may have taken, and many other things). The enzymes break down the food chemically, and the cells in the mucous membranes on the intestinal walls help absorb the nutrients from the liquid. More water is "sucked" into the liquid on the way through the intestines if it is needed to help with digestion, and the bacteria that normally live in the intestine also help to break the food down. (The bacteria also break down many of the substances that the liver secretes in the bile; this gives stool its characteristic colour and bouquet.) The undigested and undigestible residue passes from the end of the small intestine into the large intestine, or "colon", which stores the residue until you are ready to get rid of it. The colon walls absorb much of the excess water from the residue, leaving only semisolid waste (the stool).
The more unabsorbed residue there is, the more water is retained in the stool and the larger the stool is. Many things, such as unabsorbed sugar and certain chemicals, can draw more water into the stool and make it more liquid, and viral or bacterial damage to the mucous membranes of the intestine can cause excess water to enter the stool, causing diarrhea. On the other hand, there are also many things, including certain other chemicals that can, so to speak, push water out of the stool, making the stool harder. Also, if the residue is minimal to begin with, chances are that the resulting stool will be much smaller and harder.
Stool frequency, like stool consistency, also changes with what and when
we eat and drink. Some people have 3-4 bowel movements each day, while
others have one bowel movement every 3-4 days. Every 3-4 days may be
perfectly normal for the latter, but awfully uncomfortable for other people.
In practice, a constipated patient is one who has fewer or harder bowel
movements than usual and who is physically uncomfortable because of it.
(Note, by the way, that the patient must be physically uncomfortable;
whether the child's parents are uncomfortable with the child's stools
is not relevant.) People who are very constipated may also develop
anal fissures -- cracks in the side of the anus, which may or may not
be visible from the outside -- leading to streaks of blood on the surface
of the stool but not mixed in with the stool.
How to treat it (and whether to treat it)
Treating yourself or your child for constipation just because the bowel movements aren't as frequent as usual is pretty common, but it's also unnecessary and can get you into trouble. With older children and adults, you can prevent a lot of problems by adjusting the diet to increase the amount of residue, and many of the over-the-counter laxatives are nothing more than fiber supplements which produce increased residue.
Constipation is a different matter in infants, because their diet is restricted to breast milk or formula. Breast-fed babies tend to have more frequent stools than those being fed formula, although this has very little to do with residue. Among formula-fed babies, those on cow's-milk-based formulas often have more and/or softer stools than with soy-based formulas, and "elemental" or "predigested" formulas (Pregestimil®, Nutramigen®, and Alimentum®) tend to produce loose stools and sometimes cause diarrhea.
Again, infrequent stools by themselves do not need to be treated. If the stool is hard, we can often take care of it by adding corn syrup (Karo®, or any store brand), or other sources of sugars that are not absorbed well by the intestine such as apple, pear, or prune juice, to the formula. The sugar will tend to draw extra water into the stool and make it softer. (We almost never see constipated breast-fed babies; if we do, we start wondering about rare problems with the intestine.) Ask your doctor for details on whether, when, and how to use corn syrup. There are some potential problems with giving corn syrup to small babies, similar to those that can crop up with honey, so be sure to ask your doctor before using corn syrup. Also, if hard stools are frequent or becoming more frequent, let your doctor know: this can rarely be the first sign of a more serious problem.
With older children (and adults too), the first line of defense against constipation is making the stools softer by changing the diet to increase the residue, which means increasing the fiber content. Simply eating high-fiber cereals and breads can do a lot, as can eating more fruits and vegetables -- which are things you should be doing anyway. When you buy cereal, look for the "Dietary Fiber" listing in the "Nutrition Facts" panel, and try to select cereal with high fiber content. (If your children prefer the high-sugar stuff, and you can't fight it, try mixing some high-fiber cereal into their Sugar Bombs. If you're lucky they won't even notice.) Fruits and vegetables are almost always high in fiber, and so increase stool bulk: one interesting exception is banana, which has a tendency to constipate. Fiber-based laxatives such as Metamucil will also increase bulk and soften the stool, but fruits and vegetables and high-fiber cereals are probably easier to get your children to take.
I try to avoid laxatives in my practice. For one thing, some laxatives can
be habit-forming: if you use them often enough your stool will become
harder as soon as you stop, and you'll need to keep using them in order to
move your bowels at all. In rare circumstances, however, I will
sometimes recommend such things as milk of magnesia or a glycerin
suppository. Again, check with your doctor before trying any of these.
Children who are "always" constipated present a different problem. Most often these kids have had painful bowel movements due to large, hard stool. Understandably, they are scared of having bowel movements because of the pain, so they withhold their stools. Unfortunately, the longer they withhold the larger and harder the stool gets, and so their next bowel movement hurts even more. (Strangely, many of these kids have slight diarrhea, because the thin liquid coming into the colon from the small intesttine is all that can pass the large stool plugging up the plumbing.) The secret to treating this is to break the vicious cycle. My method is to use mineral oil, which is a fairly safe and relatively non-habit forming laxative. We give it in small doses at first, and increase the dose each day until the stool is so soft that the child can't possibly hold on to it -- potentially kind of messy, yes, but you only have to keep the stool that soft until your child has had enough painless bowel movements to be willing to go, after which you can back off gradually on the mineral oil. A newer laxative, which tastes better and has fewer side effects, consists of a large-molecule sugar-like chemical that is not absorbed by the body because of its molecular size and structure, but tends to draw water into the colon, thus softening the stool. There are potential risks to mineral oil and other laxatives, just as there are with any other medicine, and there are other possible approaches to the problem as well, so check with your doctor before trying this out at home.
Again, chronic constipation can be the first sign of a more serious bowel problem. Some of the (fortunately rare) causes of chronic constipation include cystic fibrosis, hypothyroidism, lead poisoning, and intestinal aganglionosis, also known as Hirschsprung's disease, in which the nerves that control the bowel-wall muscles do not develop properly. Hirschsprung's is usually diagnosed at or even before birth, but occasionally a person is born with an "ultra-short" section of intestine without nerves (the segment without nerves always includes the part just above the rectum) and may not be diagnosed until their teens or even later. Another possible -- and more common -- cause of constipation is intolerance to cow's milk protein, which can cause constipation even though it usually produces constipation less often than soy milk formulas. If your child is often constipated, she shouldl be seen by her doctor, who may run tests to find out if there is another (and otherwise treatable) cause.