M.P.
It's normal for extremities (hands and feet) to get cold at night. We adults often feel the cold and are uncomfortable. Babies and children rarely. Unless your baby is waking up and acting uncomfortable I'd let this go. She is fine with cold feet.
My grandchildren are now 9 and 12 and still have cold feet. They refuse to even put on socks. They run around in the daytime without a jacket and their arms feel cold but they insist that they're comfortable. I think kids have a greater tolerance to differences in temperature. Being cold is not a health hazard so I don't fight it.