There are several reasons. In addition, children are physically much smaller, so it doesn't take nearly as much to create a problem. Also, the nervous system of a very young child is still developing and that leaves the child vulnerable to environmental toxins. People sometimes forget that pets can be poisoned by items around the house. But there are also some specific things pet owners need to be careful of.
Many folks know that chocolate can be fatal to dogs.
On the other hand, white chocolate is safe. NOT cocoa solids. Although I have read that cats do not taste sweet things and therefore theoretically won't eat chocolate. There are other things to watch out for with your pets. Since pets are quite small, even very small amounts of some things can be toxic.