Well am I? My ex is an absolute arse.

Two issues here. Firstly, yes, your ex is an arse. Secondly, your children are not happy with you.
Your post reads as though the holiday was initially to include them and now they can't go and are having a stranger move in? If so, i can see why they're not happy.
Unfortunately when putting together the second with the first, you have ripe ground for your children to play you off against their dad. They will have gone to him with their disgruntlement and found a ready audience.
You need to decide if you are comfortable with your decision about the holiday, and if so, stand your ground. Obviously it's not grounds to have you considered an unfit parent. However he will encourage your children to think badly of you, so you probably need to prepare for a conversation with them where you sort out some sort of compensation/compromise, because in truth it is a bit hard on them.

Age appropriate truth NOW! It’s the only way with narcissistic abusers

Just give him the big thumbs up emoji 👍 They are too old now. He can’t hurt you anymore.

Are your kids actually upset or just relaying "dad says..."? Ask them for alternative ideas if they don't like your plan. Presumably your 'babysitter' will keep them supplied with food, clean dishes and school uniform so that's a couple of chores they won't have to do. They can have friends around without being landed with an out-of-control party. Perhaps agree to some later bedtimes, lax homework or several takeaways?

Ex is an arse. Your children are nearly adult so it is hilarious that he's talking about social services over the food you cook. Let him it's risible.

I hope you have a wonderful holiday

Except it isnt a stranger, its a sitter that the OP has used several times before and the kids know her.

But dont let facts get in the way of your outrage.

You do know that you can register as a childminder at 18, meaning that the eldest could potentially BE a childminder by then? I'm baffled as to why kids of this age need a childminder at all. They can manage themselves for a week surely, albeit OP should expect the house to be a bit of a mess on her return.

When my friend and I were 16, her parents went on holiday for a week. I stayed with my friend and we managed to go to school and look after ourselves quite happily. Nobody thought it was weird or neglectful. HTH.

So your original plan was for your DC to stay with their dad while you are on holiday? Despite the fact that you don't communicate with each other? What if he had other plans, even if it wasn't term-time?

I think your ex is an arse and your plan for childcare is reasonable but it seems risky relying on him to have them. Are there any other family or friends who could have the DC, perhaps splitting them up? It's expected that teenagers make arrangements directly with their father but shut down any discussion about opinions on each other's parenting.

Oh I don't miss the Good Ideas. I lost two separate kettles to Good Ideas. One was egg related, the other was instant noodles.

Use a parenting app to communicate about the kids and block him on everything else.

He has no right to have any kind of opinion about what happens when the kids are in your care as long as they're safe. And seeing as he never pays or has them it's even less of his concern.

Except that the OP made it clear that they were never going.

Kids that age it isnt a babysitter so much as adult supervision.

Four of mine I could have happily left on their own with no issues, but the others no so much. One would definitely throwing a party, one wouldnt plan a party but a party would end up happening. So no way could I have left them unsupervised, as much for their own good as mine.

This!

How do you know he used to pay the mortgage?! Making assumptions about women are we? 🤨

I would be annoyed too if i had paid the mortgage and my partner took the house and i certainly wouldnt leave my children for a week with someone who wasnt family

I've mentioned on another thread that DH and I left our now 16 year old at home for three nights when she was 15, as she had no wish to come away with us. As I said, she practically shoved us out of the door, and has since had the house to herself for a week when we went away earlier this month. 17, 16, and 15? They don't need a babysitter!

You sound like you have come up with a practical solution that won't impact his life in any way was he expecting you to beg for his help?

A babysitter seems sort of overkill for three teens (eta but to be fair you know them and their needs best, I can’t understand why your ex feels he needs to have an opinion on this!) and to be honest, I can’t see why your ex would really need to communicate with you re contact with the kids given their ages but otherwise, he’s a wanker and a shit person, YANBU. I’d be telling the kids I didn’t really care to hear his opinion about me, why are they telling you? They’re plenty old enough to understand you’d not want to hear that surely?

The fact that you booked a family holiday to a destination that they don’t want to go to during school term aside, why do they need a baby sitter? Your ex isn’t that far away in case of an emergency and you can probably line someone else up nearer a well that they can contact if necessary.

I think Pps (me too) thought the kids were booked on the hol, and because you found out they’re in school you ditched them. We see now that’s not the case!

What was the plan if it had been the hols?

Your ex sounds vile.

Or he gets up early and/or takes annual leave and/or finds an air bnb for a week….?

They're spot on the right age for a coercive controlling ex to start with the vexatious social services complaints. Young enough for it to upset you, but too old for social services to actually do anything inconvenient like insist that they go and live with him instead. No comfort at all but this seems to be straight out of the coercive control playbook. See also: texting your youngest when he'll be asleep and asking the police for a welfare check when he doesn't respond, has he tried that trick yet? He's exercised post-separation control for years by not telling you when he's having them, and now that they're old enough to begin making their own arrangements he needs to find something new.

They're also old enough for you to tell them that he can hold whatever opinions he likes but you don't want to hear it thanks.