Horshoe, I totally feel for you and have sort of been there, though thankfully didn't have to endure being with her on holiday. Firstly, YOU feel uncomfortable about it, this absolutely has to be honoured. Yes, the children need to considered, etc, etc, you know all that, but YOU are hurting and you are allowed to have your feelings and look after your feelings. Anyone saying you need to get over him moving on, need to understand why it affects you, that you should just get on with it for the sake of x, y, z, has clearly not been in this scenario to have a clue how it really feels. And the amount of years doesn't matter, you are doing your 'healing' and getting over it all on your timescale not on anyone else's and it will take as long as it will take for you to feel comfortable with the different stages of you both moving on. And right now, you are not ready for this particular scenario. And that is totally ok. And totally to be honoured. And - if you go away in that scenario, it may not be helpful for your children, because you may be distracted in your mothering by your discomfort.
And you're not talking about all having to hang out together briefly at a lunch party, you're talking about a holiday - days - and nights of having to endure being with her. And you plain and simply don't want to have her there. As much as you say the other families going are 'his friends', it sounds as if they are still yours, - and yours above being hers at this stage!, so why shouldn't you be allowed to enjoy being with them too?
And suggestions of meeting her beforehand, etc, that's not going to change a thing. Even if she turns out to be 'really nice', etc, will not suddenly make it ok or more comfortable for you. I'd say that's a non starter and not worth thinking about and anyone making this kind of suggestion has not been in your position. (I insisted on meeting my ex's new gf and discovered she was a dreadful woman, so in fact it just made life more of a nightmare knowing how awful she was - and knowing that the terrible stories my children told me about her were therefore likely not made up stories.)
I would absolutely be asking your ex not to bring her, she's got no business being at a holiday that early in, when it's both parents putting on a united front for the children. He may feel it's the right time for them to be doing this holiday together, but it's not the right time for you and the 2 of you are still in the same family package that includes both of your children, so you are still an important piece of how you work this sort of scenario out. If he wants to go on a holiday with his new girlfriend and children - he can go ahead and arrange a different one! That's very simple. This is not the one the girlfriend needs to be brought on. Which leads me on to - we all know full well that the very fact he is choosing to bring the new girlfriend on this trip, means of course he's not going to agree to not bring her if you ask. Been there. Those men are totally one track, their needs are what's most important in any given situation and they are oblivious to any one else's needs or what's actually the right / best thing to do. (And goes without saying that the girlfriend should not have agreed to go, so there's the measure of her worryingly straight off.) So - either ask, knowing he won't agree, and already armed with your next decision / thing you're going to say - or skip bothering to waste the time asking him, and know your next move now. Which I would say is not going. - And your children not going either. I know lots of people are saying let the children go with him and you spend time with friends, but why should you? If that is your time with your children, keep your time with your children. He's changed what the trip is by now saying he's bringing his girlfriend - he's changed the trip for you - but he's also (possibly insensitively) changed the trip for your children - to be one that is either uncomfortable and not as joyous and fun because of her being there - or very different because you will not now be there. He's doing that to his children. He needs to wake up to how he needs to behave. This is not about using the children against your ex, before anyone jumps on that cliche.
My ex shoved his new girlfriend down our throats, forced her into everything, which she arrogantly went along with, had absolutely no consideration whatsover for our children, who adamantly did not want to meet her or spend any time with her, let alone go on holiday with her. They had a nightmare time during their relationship because of my ex's utter lack of consideration for anyone's but his own needs in terms of his girlfriend's place in his life. It caused so many issues. All he had to do was forgo one holiday with a partner to be fully focused on his children, but he never did. And they totally resented him for all of it, with repercussions still to this day. Partly leading to them now being divorced! You could always let it go down that path! ;-)