[Big Papa Goat]

Saying merry christmas and God bless you does not neccesarily carry religious meaning, especially in casual conversation. A shirt that says Jesus Loves You is pretty explicitly christian. And it seems to me getting a shirt like that for someone elses children seems to me to be kind of like trying to impose your beliefs on someone elses children. Which is really fucking obnoxious if the parents don't share that belief.
I'd make sure of the familys beliefs before making such a gesture.