What is the enforceability of the clause in our marriage settlement agreement with parenting plan?

First I would advise you meet with an attorney to review the documents completely to make sure that you do not have any alternative options to this clause. Based on what you have written, you could be held in contempt for violating this condition. I would advise you to talk with your ex about mutually agreeing to allow contact, and if they do not agree use a local attorney to help you navigate the possibility of changing this section of the MSA.

Whenever you willfully disobey a court order, you run the risk of being found in contempt. If something has changed since the entry of the Final Judgment, you can ask the court to modify the parenting plan to allow contact between the children and the grandparents.

You are bound by the promises made in the agreement, and, if you violate those you risk the possibility of a contempt order, finding that you did not behave properly. If you have experienced a change in circumstances since the entry into the agreement and parenting plan, you may be able to file an action to modify that provision of the plan, but a great and important first step would be to talk to the ex, and see if the change can be made by agreement.

It looks like you would need permission from the other party, since that was in the MSA. Consult with your family law attorney to explore the possibility of modifying anything in a Court order.