Since the standard English pronunciation of "of" has "v" for its consonant, rather as if it were Welsh, there is no reason I can think of why a mirror should help a kid understand the difference between "-'ve" and "of". It's not that they can't tell an F sound from a V sound. They can distinguish "very" from "ferry", can't they? So that's not it.

Don't forget the optatives, volitives and gerunds.