I've only come across it at younger age youth football, such as the U13's in the OP. And in my experience, those players that want to wear a hat aren't actually that interested in the football. Nonetheless, I've never had a complaint when asking for it to be removed (and IIRC I've never come across a hat being worn that would meet the guidelines of being either black or the same colour as the shirt - so that could always be the clause to rely on).