It's not about the size of the child. It's more about maturity level of a kid adjusting to breathing and getting use to the scuba type equipment. That said your 9 year old may very well be able to handle it, but since SeaWorld has to set an objective criteria, they chose 10 years old.
Yes you can lie. I will even admit that while I've not lied at SeaWorld (they have my kids birthdates on file, because they have attended camp there), I have done so other places.