so trying to use the child to reach them isn't likely to work. Frankly they are spoiled and selfish, but you can't lead with that either. It's too bad. We could be having a much better time of it in our country if only we didn't have so many self-absorbed people so busy enjoying themselves they don't notice what's happening around them. This is my gut reaction because I'm getting peeved.
A more reasoned reaction might be to ask them to write a letter to their son, to be opened only in the case of their death, which explains why they felt so strongly that they did not need to follow guidelines and were willing to take the risk. Ask them to say goodbye in the letter, as often goodbyes are not possible, and also to help explain their reasoning so he would have that, at the very least, in the case of their early demise. This is simply a way to tell them they are being stupid without actually using the words, and just giving them the idea forces them to at least think about it, even if only for a nanosecond.
It could be they've decided they are more likely to catch it and die, so therefore, why miss out on the fun? Again, selfish and childish but certainly a state-of-mind I could see a person having. Not wanting to miss out is a great motivator.
Does their son go with them or are these couples only trips? If he does go with them, what I said above about not wanting to miss out may very well be the case -- they don't want to risk going nowhere, having no memories and dying anyway. It's a thought (a dark one tho).