My exwife moved back into the area and is now trying to get custody, which would mean that the children would have to move an hour away and leave there school after being there for two years.

Major changes, often at least two years if no major change.
She would have to prove it is in their better interest, so use the same defense perhaps that got you custody to start with

And of course any time that the court will accept their motion, you can contest the filing for various reasons if it is not done properly.
I would get your attorney taking care of it,