What year model is your Starsurfer 117?
I would try the 38 cm fin up forward in the center fin slot and the plastic 31cm shallow water fin in the back.
The real issue here will be getting the rig far enough back so that the Center of Effort (CE) in the sail can
get back behind the Center of Lateral Resistance (CLR) to allow the board to sail upwind.
Put the mast base as far back as it will go and still remain attached!
Your girls cannot just "rake the rig back" far enough to sail efficiently upwind becuase when the rig is raked back that much, it has no power.
This has to do with very tiny sails and the distance behind the mast to the CE of the sail.
It's only 2-3 inches, but the mast box is 6" or more forward to the LE of the center fin.
Small Tuttle fins should not be too hard to find. It's a training situation, so used fins with a
little damage (you can clean up the edges and the foil yourself) in the used fin/bargain bin
will work fine.
Put the smaller fin in the back of the board.
Hope this helps,