This is more a "friend" song than a "mother" song, but my mother used to sing it when I was a tadpole and we'd both cry. I only remember a few words of the chorus; maybe somebody can help. I didn't find it in the database:
I'm tying the leaves so they won't come down,
So Mary(?) won't go away.
As I recall, it was about a little girl who fell ill in the summer, and the doctor said she would die when the leaves fell. Her sister/friend tied the leaves to the tree so they couldn't fall. Anyone remember the rest?