if I were to simply promise that when your kids turn 18 I'll pay their tuition for four years at the university of their choice, and then I totally renege on that promise, there's nothing in law that can enforce it.
Not necessarily true. If you were to rely on the promise by, say, not putting away money for college, you can argue detrimental reliance under the doctrine of promissory estoppel.
Just to clarify, in this jointly crafted scenario, one person makes the promise, and the other person decides not to put money away for his kids' college fund, right? The pronoun usage(Magoo saying "I" and Josh saying "you") made it a little confusing.