That she can move her legs at will is a sign that there is no paralysis. This almost certainly eliminates the possibility of an avian virus such as Marek's.
There is a slight chance of this being neurological, but a greater chance this is a physical or motor defect. With very young chicks that...
This has happened in my flock, and when it does, I treat with people vitamins because they are much more potent. I use vitamin E in 1000 to 2000mg capsules and vitamin B-complex that has 100mg of B-2,6 and 12, all having the ability to repair nerve connections. I give one E and one B-complex...