Do you do this in hope that the children will eventually learn something? I hope it isn't vain. Good job, though.
It would be perfectly believable if there were evidence for it. However I am not aware of such evidence. Are you?
Not that I wish to support the idea of a world utterly covered in water, but... The condition is not an especally flat planet, it is an especially high sea level. One can surmise that, after the planet cooled, there was a huge amount of water settling out, resulting in sea levels so high that much of the Earth's surface was covered.
