How is she at stair climbing? That uses similar muscles. The hip dysphasia makes sense so if you still see someone about that it may be worth asking about strength issues in the legs caused by dysphasia.

Dp suggests finding a nice gentle slope that moves into a flat to help get her started and gain momentum and then build her up to longer distances.

We also used a trailgator with our girls and they just pedalled behind us for long distances which helped them with the motion.

Feeb, nothing wrong with not pedalling, just drop the seat down, remove the pedals and let them keep balance biking until they are ready - our next door dd is nearly 7 and only just started being able to pedal or ride.