Would you believe there is actually a message board devoted to The Facts of Life show!!!
this is what I found RE: Tootie..
Q. If her name in the show was supposed to be Dorothy, why did everyone call her Tootie.
A. cause it was a nickname that was given to her. Once she got older, she used it sometimes but it wasn't until the reunion that she actually wanted everyone to call her Dorothy
OP.(OTHER POSTER) Personally I liked Tootie better than Dorothy. It was a unique nickname. Plus I liked the way Nat said it when she was upset with her.