Speak to the parent, (how old is the girl) find out what she's capable of on the ground firstly - CP varies greatly from person to person, so one person may struggle to sit without substantial support, others may be competent riders.
I'd be wanting to work on her balance and coordination, and muscle length whilst on board personally. Obviously you need to use your brain in this - only do what she's capable of, have someone at her side if her balance isn't great when on the floor if you're doing exercises which is likely to unbalance her.