What will happen if a child eats old pizza?

Look out below! If too much bacteria or mold has had time to grow, stomach cramps & diarrhea may be coming. (might want to have a bucket handy, too, as it may head “north.“) many cases of “stomach bugs“ or “stomach flu“ are actually mild cases of such food poisoning from poorly kept leftovers.
Depends. Old food gathers and grows bacteria and mold, etc. Thus eating old food may lead to food poisoning or feeling ill, or maybe nothing at all. It all depends on how old it as and how much growth occurred on the food.
How old? There is no nutritional valve in pizza plus the oils can turn rancid and cause burning pain in tummy. If it moldy can cause upset stomach.