The other way you could put it is that
you want another competition pony, but need someone to ride it for you.
In Dales and to a lesser extent Fell Ponies, it is not at all uncommon for the 'producer' to not ride the pony themselves, but to have it shown ridden.
If it were me, I suspect that I would later on regret it mightily if I'd not taken the opportunity to help a young relative get going with a hobby (that could become a career - she could be an Olympian in 2020 for all we all know!
) that was in my area of expertise and interest.
If
you retained
ownership of the pony for now, by the time she was ready for her next horse you would know if any of the others were going to want to take it up, and could decide then whether this pony is your granddaughter's, for her to sell to put towards her next horse, or a pony that is now made available to the other grandkids to get them started.
How'm I doing?