I don't have much hand strength but I'm an adult and I have trouble using grip shifters on cheap juvenile bikes. The problem is as you have gathered the springs in the derailers that are too strong. Most of the responders here who are saying that she should harden up have NEVER used them themselves and they DO require more strength to use than the better units fitted to adult bikes. Seems stupid really. I've tried lubricating everything with mine but they are still heavy.

Keep an eye out for better derailers at a decent price on eBay is the best I can suggest.

EDIT: you responded first while I was posting. The twist grip shifters I had were OK when unattached to the derailer and in my case the derailer springs were just heavy, especially the front one. Its probably a bit of both. Thats cheap bikes for you.

Anthony