The DSD aren't going to enjoy a 1 star holiday if their mum has just taken them on a luxury holiday
Children are able to understand that different people have different budgets and enjoy and be grateful regardless. At least, that is how I have tried to bring mine up.
If they had £10k to spend on a holiday, the SC would be coming. The whole point is they can't afford a holiday for 5 in non term time. But the mother can afford a holiday for 3. And the father can afford a holiday for 3. And all 6 people get one holiday
Well, there is a lack of info from the OP. A PP asked what if the family don’t want to downgrade the holiday a bit? For some people £10k is a cheap holiday, surely?
All 6 people may get a holiday but 2 children are omitted from a family holiday with one of their parents.
So back to all being equal - each set of children going on one holiday paid for by their mother
Depends how you look at it. One child gets a holiday with both parents. The other two get to spend time with mum only 
This bizarre idea that children with separated parents are entitled to two of everything
The only entitlement a child has is to be treated fairly by their parents. That might mean missing out of stuff - trip to the park/picnic/cinema/theatre/ because some of the time they are with the other parent but that shouldn’t mean they miss out on all family activities all of the time. It’s not about having 2 holidays, it’s about spending quality time as a family.