If it's already in your policies and they're ignoring it, then I'd be a bit passive-aggressive with them, personally (haha). I'd say something like, "Billy seemed really hungry this morning, and you know we don't have snack until 10am (or whatever time), did he not want to eat his breakfast this morning?"
It can be hard to have a direct conversation, especially if everyone is still getting used to each other, as in "Just so you're aware, Billy needs to be fed breakfast before daycare, as that's not part of my program." and you may have to come to that, but you could always try the lighter approach, and see what they come back with.