I'm so sorry, that is heartbreaking, what an awful weekend for you and your daughter. He sounded like a lovely pony. At the yard I'm at years ago an old boy had a fall in the winter on hard frozen ground but they could find no damage then when the vet came but a few months later, first day they went out on the new summer fields they hoolied about and I assume he maybe fell again and something similar happened, I think opened up a fracture from that earlier fall but they never knew for sure.
I am just so, so sorry for you & your daughter. I hope you can take some comfort in the knowledge that he didn't suffer prolonged pain, and that he was more loved than most animals ever have the luck to be.
Goodness, what a truly awful shock, I am so sorry for you both having to go though this. I hope when the initial shock has passed that you can take comfort from so many wonderful memories.
Sweet dreams little chap xxxx