It's called hierarchical perspective, and yes, it probably is intentional, in both the case of the children and the medievals.

The kind of art you make is largely determined by your culture. In modern Western culture we use the everyday perspective that pretty much approximates what our eyes perceive. (My design book says the first painting in correct geometric perspective was drawn in 1427, by some guy named Masaccio.)

Other cultures used different kinds of perspective, such as the Asian 'reverse' perspective, which "prescribed convergence of parallel lines as they approach the spectator"--rather the opposite of what we're used to.

Anyway, my favorite perspective on small children's artwork was in Terry Pratchett's "Hogfather".