Plan some stuff for you to do the week that they're away and treat it as a holiday for you as well.
Catch up with old friends, go out dancing, see a comedy show, go to the cinema, eat whatever you fancy and revel in the fact you'll only have yourself to clean up after and cook for a week.
Let your ex do all the holiday shopping, including clothes, swimmers etc, literally hand the kids over with a smile and a wave, don't do any of the planning or thinking for him.
I have to say I think the new GF is going to be in for a shock. A "holiday" with a 4 & 7yr old (who aren't your biological children and you don't know very well) and your "in-laws" is my idea of hell.
Your ex sounds like a lazy-man child who can't be bothered to look after his own kids and hence sees them when his gf &/or parents are around to do the bulk of the parenting.
If your ex's gf is using annual leave and her own money to pay for her share of this trip, she's going to want a bit of a break and I'm not sure how much downtime she'll actually get; even a flight with two small children can be shattering.
I'm betting that the vacation will give your ex a chance to wave some of his red flags at his new gf when he leaves the bulk of parenting to her and the GPs.