I like the idea of it occurring in the minds of those who thought it up because its still possible a new theory could replace it. Thats the joy and strength of science. But it is not very good for explaining to children. I think the best answer would have been

We are not sure yet, which is why we need young, fresh scientific minds to do well in school so they can help us figure that stuff out later

More important to get them interested in learning than to get them impressed by you.

(edit to add I dont mean anything bad to you by saying that last line. I hope you are not hurt.)