There is nothing wrong with selling an old horse to someone who will love and care (and ride). If horse likes it's job (and most do), riding will make him going longer.
As already suggested consider 2-3 months of full lease (so you can check on how they ride, how they care for him, if he's a good fit for the little girl, etc.). If it'll work out then you can just sell him and visit him from time to time.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."