Dd is 7 and a few times while we've been shopping asked for the short t-shirts/jumpers that sit above the belly button. I don't like them and have said until dd is older that I don't want her wearing them. Dd is fine with that and hasn't pushed any further.
Dd was out with MIL on Saturday and she bought her one (not realising it wasn't just a normal t-shirt) she is happy to return it and swap it for something else. But AIBU. I couldn't say exactly why I don't like them, but they look to old on little girls.