He has had to change in order to do what he's done. Like I said previously if he allows his guard down, the whole reality will hit like a ton of bricks.
Yes, your ds may be over dramatic and things might not be as bleak as he says. Thing is, you KNOW your kids, you've got their backs always. If I can offer advice about this then I would suggest that whilst you're on your lovely holiday you talk to them about how they really feel about staying at dad's. Don't minimise their doubts, but likewise don't make things more than they are. Just talk, try and take the emotion out of it, and if they don't want to stay with him, her and her brood for a while, then take their reasons on board, and agree that they really don't have to, then when you get home email him, telling him their reasons and for now there will be no more stopovers.
The key is that you follow their lead: if they, on balance, want to continuing staying with them, agree, let them go, paste a smile on your face, and step back. Don't ask too much, keep it light and laid back. If they don't want to stay, then support that in the same manner. They will hopefully have a good relationship with their dad, but like I said before, he and only he can drive this.
Remember too that a couple of years will make a massive difference: they will decide for themselves, they will see him in a different way as they grow up.
You have done, and are doing, bloody well. You are their rock. When he walked away, he gambled his relationship with his kids. You never took your love and care away and never will. They are lucky to have you, so just relax, because whilst they might not be happy with him, they are safe. If he doesn't feel ready to man up and show his love for his kids it isn't because the love isn't there, it's because he can't show it. He can't show it because it forces him to acknowledge how much he's hurt them.
Enjoy your holiday, every minute. You deserve a wonderful time x