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.
Stairs she is ok, but larger height than normal stairs she seems to struggle with - compared to DS. I am sure she needs a follow up this year about her hips so I will ask at that if no improvement. She doesn't seem averse to wanting to practice when she gets chance (and we use the carpark in the doctors across the road on weekends which has a little slope) but so far still seems like such an effort for her, but she at least seems willing to persevere.

It is her first pedal bike, she wasn't interested in a balance but now might be because a girl down the road zooms round on one. So I might take the pedals off it and let her do that for a bit. She is not fantastic on a scooter but not bad. She will walk a long way up and down hills etc without problem and I would have thought hip issues would have shown there too - but it still niggles me that might be hips.

I think I will try a trailgator, she loved the seat on the back of the bike before she got too heavy. I think her heaviness is part of the issue as she is too heavy (23kg) for the strength she has at the moment, am sure that will balance out.