They would have to pay for the general care and exercise of a horse too because not only are they away too often to do it themselves, they couldn't be bothered to go out often enough to do it, unless the horse lives in b&m then it might get frequent visits.
Not one of them can ride a horse, so there would be lessons from beginner for every child that wanted it.
Let them get a horse (as long as it's in proper stables with someone else taking care of it), it will soon drain their remaining money.
I doubt they are though, even Sarah knows that it would involve too much exercise and too much time away from the sofa(s), she'll just buy some horse decorations if she sees them in b&m for yet another bedroom makeover theme.