I also recommend putting the money in an online 2 year CD or savings account. You don't want to tie it up in a 529 in case none of your children will need it.
We are contributing $20K for our niece in college, paying $2,500 per semester after grades are posted & if she has at least a 3.0/4.0 GPA as a full time student. We have an online account login at her university & send it directly there for her tuition.