Yep, been playing a while now and I agree. There is no such email. What you got was the notice that you had fulfilled the trophy that is Four Is Enough. Even if your adopted child had actually been born to them, you still would have received that trophy message because as soon as you have four kids OR FIVE then you get that message to let you know that you accomplished getting that trophy.

It has nothing at all to do with the fact that the child was adopted.