Even at the highest levels allowed by the FDA, dietary ammonia is unlikely to have any noticeable effect. The tens of milligrams of ammonia your child would get from a large serving of cheese is insignificant compared to the ammonia produced by his own gut flora. And it's truly harmless compared to the the toxins he gets from an intestinal microbial overgrowth (such as a pylori infection) which may very well be the most prominent cause of intestinal disorders.

Even at higher levels than those allowed in food, I've seen no evidence that ammonia has any intraluminal effects beyond smooth muscle hypertonia. So even if your child somehow ingested more significant amounts of ammonia, for example from cleaning products, the worst symptoms will be some cramping. Note that I'm not talking about ingestion of concentrated ammonia, which is a corrosive hazard.

Think of it this way: if your child really was affected by dietary ammonia, you'd have to avoid a lot more than hard cheese. Foods containing more ammonia than pink slime include nearly all cheeses, cured meats, peanut butter, onions, mayonnaise, and others. If ammonia really was a problem then we'd all be pretty screwed.