Your obligation today for contributing to your children’s college education is dependent upon your divorce judgment. If, in your divorce judgment, it provided a resolution and a formula for the contribution to college, then follow that order. If it did not, the issue at hand would be did you retain the jurisdiction of the court to deal with that issue at a later time? If that is a part of your divorce decree and you cannot reach an agreement with your former spouse, you can file an application, a motion to the court, to have that decision determined.
If, in the alternative, you did not retain the jurisdiction of the court, and your divorce decree is silent with respect to allocating responsibility for these finances, you have no recourse. Your only alternative is to reach an agreement with your former spouse in this regard.