it sounds like you have a great idea so far! i really like the fact that you want to keep your horse in some work even after shes "retired". i would see if anyone at your barn is looking to lease - if there is a trainer, then maybe one of her students? or even a friend or child of a boarder or the barn owner? if not, then you can start looking outside of your barn. word of mouth is always the best option. i would also post something on a local facebook group page or craigslist or something. i would suggest doing an on farm lease, that way then you can still keep an eye on what is going on. if you do an off farm lease, then make sure its close enough so you can keep an eye on the horse. also, if its a younger child, you may want to help them with there "new" horse, or have them under your barn owner or trainers supervision.
as far as expenses go, that really depends on the situation. if they are doing a full lease, they pay everything - board, farrier, and general vet expenses. if its just a half lease, then they should pay half of board and maybe half of farrier, but thats as far as i would go. also, if its a half lease, i would expect them to use your things (unless if they already have a saddle and stuff that properly fits your horse). if its a full lease, they or you can decide to get there own things for the horse or not. if it is off farm, they are expected to pay everything and buy there own things.
make sure you have a solid contract that is read and understood by both or all parties involved. they do have some good examples online of what the contracts should include.