One of the other answers might have been a bit harsh and would lead more established writers to mutter sarcastically "wish I'd thought of that", but there's something useful in it - it sounds like you're thinking ahead to publication before the book is finished.
While you'll want to consider your intended audience during writing and editing, an important step is the book. I've worked in manufacturing, and while market research is significant it's best to have a product when considering a sales strategy.
At this point I'd concentrate on writing the book. Worst case : no publisher takes it. This still leaves you with something you've achieved, and there are other options like self publication or returning to it later when you're more experienced.
You're a lot of the way there. You've established characters [the girls], a premise [the riding group and competition] and a plot. You've asked other people for advice.
(As an aside - and I'm at risk of going off-topic here - the character that fascinates me is the one who was injured and won't be able to join the competition. I hope you'll cover her views.)
It looks like you're doing it right so far.