If somebody bought your baby (12 months) a T shirt carrying a band or singer YOU the parent do not like, would you dress the child up in that shirt in public?
I have found an Aladdin Sane baby T shirt which of course is neither embarrassing or offensive but the parents are not fans at all of the great man.
id imagine that seen as you are buying for their baby that you are close so yes id buy the tshirt then just make the joke that you just did about making sure the baby gets into decent music.
I wouldn't bother Wardy - they'll either only put it on the baby when you come to visit and he won't be in it long.
If you bought it for me, well, I'd use it to dust, sorry :)