Divorce Support Forums: College expenses agreement

Indiana divorce 10 years ago, NCP. Divorce settlement did not specify college expenses, only a note stating that both parents would like children to attend college and it could be discussed at a later date. My ex is grumbling about getting an agreement in place. My daughter is 15 and will be a freshman in high school this fall, son is 12.
Any advice? Anyone have a good agreement in place that is fair and does not allow the mother to "bully" the children into staying close to home to attend college in order that she can also request CS while they are in college?
I want my children to make a smart choice as to where they attend college and I am willing to help them, but really do not want to be paying for them to continue living in the shadow of their mother, especially as I know that she will only do so to retain control of the children for as long as possible in hopes of extracting the maximum from me financially.
Any advice would be appreciated. Thanks.