If you signed a note to her for the money, I would have her mark the note paid in full, and sign it. If there wasn't a note, I wouldn't worry about it.
I'd thank her, and I would put the extra money in a savings account or a savings bond for J...a college fund. I would tell her what you are doing, in writing, and follow through.
Under no circumstances would I trade this money against child support, this is J's daddy's responsibility, not his Mom's.