It's interesting, but I do have the 'Little Mermaid' association. However with the child's classmates that might be less strong in that generation. I loved the
Ursula Le Guin books, so that's a good association for me.
I love
Ulla, so if I were to use
Ursula,
Ulla would be a permanent nickname.
Ulla pronouned as "OO - luh" (the OO sounding like in 'cook' not 'spoon')