I don't think I ever "believed it was flat", as such; rather, I'd just never thought about it. At some point (before I started at school) I learned it was round, and asked the usual questions about why people in Australia didn't fall off, but this was more a new bit of knowledge than a contradiction of an old bit of knowledge.

Matthew's the same. He asks about different countries and we always show him on a globe rather than an atlas, so he knows (at some level) that the earth is round.