Cant play with iPad using Game Center

I bought this because my son has it on his iPad and really likes it, so we could play cooperative mode together.

It is bad enough that you need to pay twice for the exact same app on your phone and your iPad, but then you cant play together via Game Center as the game advertises. (I have not tested playing together via Facebook as my son is still small and so he does not have an account.

Hekansjkw about Bloons TD 5, v3.12