If you are a grandparent, it's probably been a couple decades since you have been a parent. Things have changed... A LOT. All one can do is do your best as a parent, set a good example and hope for the best that some of it sinks in. Unfortunately these days, that doesn't always seem to be the case. Despite how much "fear of god" you think has been instilled in them. Or how much you think "dishonoring the family name" matters to them (it doesnt...)
I am thankful that Wyndham has a very strict "no under 21" policy, and honestly wish they would raise it to 23 or 25. I have one, and soon to be 2 kids who could use it too. You can't rent a car until 25, for instance. And my experience is that in most resorts (especially ones without any sort of guard gate) the resorts do very little policing of those entering the facilities. And especially at beach resorts, this sort of kids/party nonsense happens way too often.
Of course this is just my opinion, but you will find, at least this time, most owners will side with me on this one. Just trying to provide as much info as I can, along with my opinion, which you can take or leave.