B.C.
I know this may sound like an odd suggestion....but hey I have been in some odd predicaments myself...why not look for a toddler sized sleeping bag and zip her in? or if you know someone handy with a sewing machine just make a blanket that won't come all the way untucked, it would not have to go very far up just far enough it won't slip off the bed. ( My daughter is 8 and her comforter ends up on the floor every morning ( she sleeps in flannel jammies )
No I think making two or three trips a night to pull covers back on is a bit on the wild side, but its tough negotiating with a two year old = )
Lots of luck ( hope you don't think my solution is too wacky, but to me sometimes you have to think outside the box LOL)
B.