I went on a trip to Florida with my boyfriend's family when I was 20, organised probably when I was 19.
All parents involved were happy with us being part of a family holiday because we were high school sweethearts style, four years, they all knew fine well about physical intimacy and lack thereof as both families chatted about it with us and each other, in making sure they had the same rules for us in both homes and respecting all parents.
We had also both become part of the others family, very close with the siblings, shared Christmases, my parents went to grown up evenings and parties with the other parents, they were all friends and had just simply met through us. They all expected us to get married. They all got to know each other and spent time together without us as well as with us, so there was plenty of understanding exactly who your child was spending time with, as well as what the parents would like in terms of behaviour.
Even then, at 19/20, for this holiday, my bfriends mother spoke to my Mum way in advance, said I was welcome to join and they'd love to have me, if my parents were ok with that. I was 20! She spoke to my parents before mentioning it to me or my bfriend and his sister, who was like my sister as we were close. Both would have and did say "Hell yes, we want Lu to come, so cool!" and we were three overgrown kids in Disneyland together:)
I paid for my flight (20 years old, would be a bit much for me to ask my parents), the family said "Well we have the villa, the hire car and the family passes anyway", there's no difference in costs, so just your flight is fine. I and my parents wrote thank you cards and bought gifts to say thank you afterwards, it wasn't expected, but a spa trip, a nice bottle of whiskey and a pretty necklace (for mother and sister who had shared her family time too!) was just thoughtful, especially as they had been generous in taking me, in a respectful way as if I was of their own! That's what you do when you take a young person on a family holiday with you, they are still the same age as your own kid and need to feel that way.
They also had family trips without me, which I always thought and think is important. I was part of the family but it was still important for them to have time together as their own family as they had since the kids were really little and as they were getting older, maybe they'd be losing the group of four of them before long so it was nice for them to have that time.
That was at 20 years old. That's how parents should be behaving towards each other:)