I have been the XW in this situation. My XH left the country on at least 3 occasions for 2-3 weeks with his now-wife, and chose not to tell me each time, on the basis that I could email him and therefore had no right to know anything about his life. At the time, he never called or skyped our DC anyway so going 2 weeks or so between any phone or physical contact wasn't unusual. Personally, as the RP this lack of contact with our DC was a contributing factor in me and XH having a really poor relationship back then. (I was pissed off that he wouldn't make the effort, not blocking him from trying, to be clear.) Is there phone contact with your DP and his DC? If so, won't he be skyping or calling from a hotel/abroad and therefore his XW will simply see you are away anyway?
Anyway, for all those who think that it'll never happen that XW will need to get hold of him, I disagree. Sod's law. On one of the occasions my XH went on holiday, our eldest son (then still only 2yo) was hospitalised. I was home with our baby at the time and naturally tried to let him know. I couldn't get hold of him, he was far too busy/complacent to check his emails, as you tend to be, on holiday.
To tell you the truth, it didn't matter in the immediate situation. I had other family to rely on and my DS recovered, thankfully. BUT and it's a big but, I have never since then looked at XH as someone I would EVER rely on, turn to or expect anything from in case of emergency. In effect he's been demoted in my mind from next of kin to someone on the 'should be informed' list. I know this will have a mixed reaction on the SP board but there you go. Cause = effect. He appeared to give no shits whatsoever, and convincing me otherwise is going to be pretty hard.
I am perfectly pleasant to him and facilitate his contact these days exactly as I did before, without any drama, but the emotional legacy is there. It's just one of those things that you clearly instinctively know anyway: there are lasting effects of treating the childs' other parent as the adversary, and in this case I think you're right, it's unnecessary.
If your DH has already got a bad relationship with his XW, she may already feel unable to call on him. Nevertheless, bear in mind that he won't win anything by living up to someone's low opinion of him. Bridges are built by the bigger person, not the one who wants to stick two fingers up at their ex. It won't change anything about your holiday to say you are going, and provide contact details. So personally, given my experience, I would say you should.