OK so you have to be realisitic...
My thoughts - sit with kids, tell them that if they want the parenting arrangements changed you will do what you can to make it happen. Be realistic with them, tell them about the complications delicately... And do one more thing - tell them not to answer you straight away, unless they want to...
Tell them that you want them to think about what they want and you will see what you can do to achieve it.
Look is living with you majority of the time also possible? If so, you can go to mum and say is she is gonna make it hard on the kids, they just might choose to stay with you guys...
Last thought - rarely do people do things like this without having pressure applied. Asking for more time ain't helping, unless it suits her.... So you'll have to apply some pressure to get her to become more flexible.