I've known a few small business owners who have done stuff like this after adults complained there was nowhere to lock up their adult bike because a kid bike was occupying the space. Basically the small business owners' responses were "you're in shape. There's a large rack around the corner. Your bike is important to you, but that kids bike is their entire world right now. You can shut the hell up and walk"

And then put up signs like this or that just say "kid bike lock up only."