My four year old isn't even the one riding and I still went out and bought her boots / helmet. Why because she likes to help "groom" Casper which normally involves me holding her while she uses a soft brush on his neck or shoulder area. However I want that helmet on her any time he's not in her stall but she's within 4 ft of him. It's a good habit for her to learn now, we also make her wear the boots to the barn each time another good habit to form now. We do let her feed all the horses treats though except the nippy mean one. We've taughter how to flatten her hand and let the horse take it off her palm.

edited to add: I think some of the stuff I see others let their kids do is crazy. These will be the first ones that blame the horse not themselves.