Food is a whole different situation. As with most animals, when food comes into the picture, the scene changes. As far as kicking at him in general, I'd say she's showing disrespect & she feels she's superior & she thinks he should feel that way too. In safety for the boy, I'd keep him away, or else take some serious time to school them both on his superiority. That is, with you there, let him lead her, back her, whoa her. Again & again. He does seem a little young for this (IMHO) but you've got to remember, she's a horse, just 'cause she's little, doesn't make her different from other horses.
She clearly has no respect for him. He can't show any inferiority, or being a horse, she will take advantage of it as she would with any other horse. If there's two horses, one's the boss, even if YOU don't realize who it is among them.
I wouldn't put up with the swinging butt thing either. They make a little hand-crop, you know? Use it. You've got to show these girls who's boss mare -- YOU & any other human who is around them. Sorry, I just wouldn't tolerate this. I don't mean to sound like a know it all. I've just been around horses (big & small) a real long time.