I think the point is that they're beating a dead horse. It's not their place to keep pushing their opinions on you. They voiced their opinion once. Fine. Friends do that. But they need to drop it because it's none of their business.
I'd say something like "This subject isn't up for debate. Please, drop it."
If there is silence after that, then you can bean-dip and say "Now, who would like some tea?"
But if they counter with "But, you're robbing him of the opportunity to..." or something where they're trying to continue, I'd say "I mean it. We're the parents. We've made the decision. It is not your place to try to convince us otherwise. We're not discussing this." I would never JADE (justify, argue, defend, explain) unless you want to get into a debate.
It's not about the matter at hand, it's about the fact that they're sticking their noses in your business and it doesn't matter what the subject is. It's not their place to tell your family how to do things.