Were the children right to intervene to prevent their elderly father from divorcing their elderly mother, given their intention to sever his communication with her completely?

Striving for reconciliation is a good and commendable act, especially when it concerns one's parents. You have done well by preventing your father from divorcing, and you must exert effort to prevent tension between them. If your mother is ridiculing your father, she should be advised gently, and assistance should be sought from someone she respects.