Okay, we were in a similar situation with mum demanding we spend $900 to change flights that had already been booked and paid for because they were an hour later than her preferred departure time.
We didn't change flights, and I don't think you should, either.
You're on the cusp of a hearing. If mum withholds the kids when you're only otherwise seeing them for very marginal periods in a year, simply because she felt the flight times you'd organised "weren't fair", then it's not going to make her look great. Your time with the kids being an inconvenience to her really isn't a good enough reason for your time with the kids to be limited even further.
If it were me, I'd tell mum that I'm looking forward to seeing the kids on (date of arrival) and that I am sorry about the return flights and I will see what I can do. After they arrive, I'd tell mum that I couldn't change the flights because the cost was too high, but would be happy to do so if she would like to cover the cost. Then I would put them on the plane home as scheduled and get on with my life.
We didn't change flights, and I don't think you should, either.
You're on the cusp of a hearing. If mum withholds the kids when you're only otherwise seeing them for very marginal periods in a year, simply because she felt the flight times you'd organised "weren't fair", then it's not going to make her look great. Your time with the kids being an inconvenience to her really isn't a good enough reason for your time with the kids to be limited even further.
If it were me, I'd tell mum that I'm looking forward to seeing the kids on (date of arrival) and that I am sorry about the return flights and I will see what I can do. After they arrive, I'd tell mum that I couldn't change the flights because the cost was too high, but would be happy to do so if she would like to cover the cost. Then I would put them on the plane home as scheduled and get on with my life.