Horses in the wild will groom themselves by standing face-to-withers and "nibble" each others whithers and back. If your horse is doing this in a general "playful" way, he is probably just telling you that he is enjoying his grooming and would like to reciprocate by "grooming" you back! However, we can't let them groom us as if we were horses because of course our skin isn't as tough. You need to gently remind him that you're happy to brush him without needing to be "groomed" in return by turning his face away or simply tying him tighter. If he has his ears pinned at all during this or looks annoyed or agitated, if he's dancing around or twitching/wringing his tail, those are signs that he's very unhappy and this is not a "lets be friends" statement. Take this time to help your daughter learn how to read a horse's actions and behavior! Great learning tool. Check out some books at the library on equine behavior. The more you know, the safer you'll be! Enjoy your friend!