Going on a journey to work with someone else's horses is the big issue,
because it means she's now unavailable for this weekend, when you say the roads are totally clear
This girl and her parents clearly don't regard this as a proper job, just a bit of fun experience
so it's well down on their priorities
She might be a good employee in a few years when she's matured, but not now
You and your horses need someone who can take this as a proper job for the hours agreed
Many young employers won't have a car, but you need someone who'll only be absent on those days when ill, or if conditions are unsafe,
not also on unnecessary extra days.
No employer need accept that.
If you had other suitable candidates, I suggest you contact the best and ask if they are still available to start soon.
I wonder if what persuaded you to take her on was the fees for having another horse on your yard ?
Not worth it, if so.
Don't mix the role of employee with customer.
On the days she or her parents have something more interesting, you'll be looking after that horse too
You'll be checking up on it at night when it's ill - because her parents probably won't drive her over at 3 am
Very awkward to sack someone, even during the 2 years period, if their horse is in your yard