If you wanna get technical/fancy, you can use messieurs (abbreviated Messrs.; Messrs. Thomas).

But it would be the Misters Thomas or the Thomases.

For a kid, I suspect he might just say "Mr. Thomas and Mr. Thomas" or "the Mr. Thomasas". Are the two Thomases related? Maybe "the Thomas brothers/cousins/whatever" if he's talking about them but not to them.