So did the agreement that you signed also say anything about the rules around when you see the kids?
Is there anyway you could move to where she is? I hate this, but in reality, moving closer to mum will make this easier. Even once you get court orders that say she has to contribute towards making sure she lets you see the kids.
You could still find yourself in a position where you go to the airport, only to find out the kids missed the plane. It will never ever be that she was running late and failed to get kids to the airport on time... It will be the 'kids' who missed the plane, because she will take no responsibility.... Eventually it just becomes a pain and there isn't really much a court can do unless you are prepared for a hell of a struggle