My pil like to take their dc abroad when each one turns 40, the pil pay for the entire holiday for them and their dc (4 dc all in their 30s/40s) the last holiday was 2 weeks in Greece when sil turned 40.
It is very kind of them to pay for their dc to go away but there is never the mention that partners or dc would be welcome but obviously would have to pay for themselves.
The last time dp went away it caused lots of issues between us mostly because he spent a large chunk of his annual leave on this holiday, we live in dp'a home country and at that stage I wasn't very settled here which made spending 2 weeks alone worse.
Dp turns 40 soon, we will have 2 toddlers when he turns 40, aibu to say if he wants to go away and celebrate his 40th without us he can take the dc with him as they are as much his family as his parents and siblings.
I just couldn't imagine organizing holidays for just my dc and dp 20 years after they leave home, surely it's important to embrace the way the family has changed.