One of the possible symptoms of CP is being underweight, because the spasticity of the muscles makes the body have to work a lot harder to move, which uses up a lot of calories. My mom had the same concerns when I was young, she would beg me to eat more, but you're just not hungry anymore after a certain point.

So do not worry, you are doing as much as you can but she will probably just always be underweight... I'm 5'2" and weigh about 95 pounds...maybe she will gain more as she gets into her teen years.