If Chelsea can afford to send them to private school, why wouldn't you. Also the right school isn't all about league tables. There will be families who want academic hot house schools, those who are more interested in sports facilities or drama, art. Those who want full in boarding, those who want flexi. There's a lot of families who can't or don't want to send to top boarding schools but would rather send to a good local private school where the child can still have a home life and they have less fees. I imagine they value a more local school so Harlow still has time for the horses, shows and vlogging.
I feel sorry for Harlen sometimes, it's obvious Harlow and her Mum are very close, doesn't appear they have much if any relationship with their Dad. Who does the "boys" stuff with Harlen. Seems like he's left at home a lot whilst they go off with the ponies. He probably only gets such enthusiastic praise when it comes to the riding. He doesn't seem interested and can't hold the reins. They are probably trying to keep him interested by jumping as he'd soon be bored working on the basics and wouldn't want to ride at all.