As with geekette8 I don't think it was something I thought about one way or the other at an early age.

However, unless my memory is tricking me, once I found out that the Earth was round I did have a short period of thinking that we lived inside it (as on the inside surface of a shell) and wondering how they managed to get the rockets out of the Earth in order to travel to the Moon. (Although it apparently didn't occur to me to wonder why we could see the Moon in the sky.)