What is the custody schedule right now?
Counseling would be good for the children, getting a trained and experienced counselor's
objective perspective would be very
validating for them. You too can be
validating and
a problem solver.
Sadly, laying low is not a [good] strategy, at least not by itself. It makes sense not to trigger the difficult parent, but avoiding an incident may not be the best solution long term, it's just as likely to enable and even encourage an exaggerated sense of entitlement and demands.
It's not like they're in grade school, they're both older children, once they're driving, they can more easily 'vote' with their feet. The older they get the less likely court is going to 'force' them to comply with a schedule or order.
Are there any arrangements for the court to listen to the children's wishes? For example, they could have a Guardian ad Litem (GAL) assigned. You may have a lawyer, your spouse may have a lawyer, why not the children? Court may not do as the kids want, of course, but knowing how they truly feel may sway the court to a certain extent. However, if you're seeking a settlement and not a court decision, just a rubber stamp from court, then that may not work.
Once you have a court order, whether by settlement or decision, it's much harder to change.