I would keep it short and sweet. I can't remember your exact post, but something along the lines of this should work.

Dear XXXX,

I just want to share that after XXX weeks, XXX is still not settling in here despite my efforts to calm him. After careful consideration, I have decided that I can no longer provide care for XXXX after this date (whatever notice period you need to give).

Sincerely,

Bri