The posters who have been through this themselves have mostly said that they would have had the worst holiday of their lives if the parents had told them beforehand that they were separating.
IMO we are often too quick to attribute adult emotions to children. Children often live in the here and now, and if their parents are seemingly together and they are a family then the children will accept that far more than if they’re not together and then pretending to be a family.
Whenever parents tell their children they’re separating there is a risk that those children could think the last few weeks/months were a lie as they were oblivious.
When me and my H separated DS genuinely had no idea as we were mostly on speaking terms and any arguments we had were after he’d gone to bed and they were never shouting screaming matches iyswim. He has since confirmed this since he’s got older.
If we’d gone on holiday after separating though he would maybe not have been confused but would definitely have harboured feelings of us getting back together.
If you can manage to be civil enough to go on holiday and for your DH not to move out straight after you come back I would definitely wait until after the holiday to tell them.
FWIW we lived in the same house for 9 months after splitting because of finances, we had separate bedrooms and did some things with DS separately. We told him about 4 months before we split, when I had my eye on a house to move to. But that’s far different than if we’d been on holiday together and stayed in the same room.