Is their a flag that marks a NPC as a child, and if so how can you change it?

Not that I'm aware of. The children are tagged that way because they're designated an entirely different "Child" Class, with it's own stats and textures that go beyond just a simple tag.

Children are separate races in Skyrim. There is a Breton Child race, an Imperial Child race, a Nord Child race and a Redguard Child race.

There is the child flag that basically flags them as invunerable if thats what you mean (it also affect other things like forbidding them from using animations that are designed for advlt NPC's so they don't scale to giant size). Its only removable with the CK, xEdit or a SKSE script.

If you talking about races then they're just another race like any other i.e. Breton Child Vampire race etc.