If the XS don't fit, try the Junior Fins offered by Aqualung and Akona. They are exactly the same as their Adult versions. Sometimes just changing the type of bootie you wear may help in finding a pair of fins that work for you, as fit has a lot to do with the shape as well as the length of the foot pocket.
Also, if your straps keep sliding off, try spring straps [get the narrower ones].
I am a US 5.5/6 UK 4 and wear a XS Cressi's.
Good Luck.