We all force - or at least, urge - our beliefs on our children. It's part of parenting. As they get older, hopefully we're open to hearing their beliefs, too.
Just as toddlers can't read, they also can't understand the beliefs they are being required to project. But others can, and some adults and other children will treat the babies differently because of the words on their shirts. So I don't think kids should wear any political message (which fundamentally this is), even in most cases if they've chosen it themselves.
Meg learned this the hard way as an older teen. The liberal stickers on her VW made her a target in the small town we lived in. Her car was secretly sabotaged (lug nuts loosened) and we had to take severe precautions. She was outraged that she was asked to "suppress her beliefs" in her words ("tone down the car" in ours), but her safety was paramount. She's still an activist, and we're proud of her, but much more aware of the impact of her public beliefs on her kids' relationships.
Of course, I did get DGD a "Make Art, not War" tee - but that's pretty innocent, don't you think?