your 16 year old clearly would have a say.....your twelve year old is a little young, but that does not mean the court would not take the 12 year olds opinion into consideration for at least what type of visitation there should be.
I would file a motion to modify and seek appointment of an attorney to repesent the children as well.
Courts do consider the children's wishes as a factor in any visitation or custody dispute. As a sixteen-year-old can file their own custody modification proceeding, their wishes are very clearly considered. In addition, both of your children may have an attorney appointed for them that may be able to present their views to the court. It is always better to proceed with a request to the court for a modification of the current order then simply to violate the current order as it exists.
If you do not have an attorney, I would be pleased to discuss this further.
Fred A. Cohen, Esq.
This response is intended to be a general statement of law that should not be relied upon as legal advice and does not create an attorney/client relationship.