I am so sorry to hear of your ongoing pain AmandaKDT
I think it's simply a short and to the point letter that should be given. That due to health reasons you are reducing the number of children in your care at any given time. I wouldn't hold yourself to restricting ages for the future by adding that. I think that your reasons are totally justifiable and I can't imagine this family not understanding. I wouldn't worry about recommending someone, although nice, it's really down to them to find someone and you don't need to burden yourself with that task when it isn't your responsibility. If you already had a provider you were friends with and who provided back up care then that would be different, but I'm always a little wary of recommending a provider I have heard of but don't know personally.
Good Luck with the lego job, sounds awesome!!