Lactose intolerance is a kind of enzyme deficiency most people are familiar with. If your body doesn't produce lactase (the enzyme used to break down the sugar in milk) you will get sick from milk. The genes that turn on enzyme production for lactase are usually turned on for babies sometimes diminish depending on a person's genetic make up. If you are lactose intolerant, you can either: avoid milk, drink lactose free milk or take lactase (the enzyme needed to break down the lactose) when you are going to consume milk.
Many people suffer from non life threatening allergies and intolerances and need to avoid foods in order to be well. Enzymes can allow them to reintroduce these foods into their diet. In fact, some parents sometimes report that their children do better on enzymes with a broader diet than a more restrictive diet. If you have an IgE allergy--like a peanut allergy--enzymes would not help you. But people with IgG allergies and food intolerances often find enzymes helpful.
My son has taken Tri-enza and No-Phenol by Houston Enzymes for two years. They enabled him to break down the food and absorb nutrients and allowed his gut to heal. They also allowed him to eat some food that he was sensitive to. Because they are replacing enzymes that are already in your body they are a great thing to try for kids who have digestive issues such as diarrhea or an inability to completely digest food that they eat.
Here are the enzymes to break down food--they are available in chewables for kids as well as capsules.)
AFP-Peptizyde breaks down Casein (milk protein), Gluten, and Soy.
Tri-enza breaks down all proteins (gluten, casein, soy, dyes, lactose etc.) This product is a combo of several other products
No-Fenol breaks down fruits and vegetables and good for those who follow the Feingold diet and avoid dyes.
I recommend calling and asking for samples before you invest in a whole bottle of any supplement. Enzymes are one of the supplements I really think is worth parents considering.