I remember a rule that was something like reduce sections to 75%, and reduce the size of the object by 50%. But I forget what the actual numbers are. And obviously it will vary as to what your idea of a "Child" might be. But the idea is that if one had to cut the leg of a Windsor chair down to a half size, half thickness legs don't look good, and may not take the abuse even if though children are not all that heavy.
Sometimes weight isn't important and one might make a table half the size and keep the dimensions since invariably some person comes along and uses whatever one makes as a stepladder.