…although I think Hadyn is right, these are in fact slugs, and not flatworms as I had originally thought:

Bella found this pair of beasties under a large dried leaf (left over from the summer when the girls were using it as a sun umbrella). As far as slugs go, I pronounce them pretty indeed, with their semi-transparent leaf shapes and contours.

But what were they up to under there? Hmmmmm.