To say if DP doesn’t take time off, SC have to come less?

Don't be silly, she can't do nothing.
He'll go to work, the kids will arrive and she'll be stuck with them (trying to wfh).

Of course she can still do nothing. If they need something they can ring their dad.

100%
I completely agree with you OP.

he only seems to have them in the holidays and one evening? I have asked the question so perhaps missed the right answer

Totally sympathise but not sure what you can do - other than find somewhere else to work during the holidays and make it clear to DH that you’ll be dropping yours off at nursery then heading to the ‘office’, so he better have made arrangements. Then stick to it.

He’s reluctant to put his children into child care as they aren’t that keen, yet he’s perfectly happy (even determined) to engineer things so that it falls by default to you, who also feels unenthusiastic, to take care of them!
Why should his children trump you in this way?I think it’s an outrage.
He is comfortably poised between two women, one of whom is their own mother, but he dumps them in your lap. Feck that.
I do not understand the rules of stepmotherhood in circs where the bio mum is alive and well and nearby.
Why must you shoulder such responsibility?
If he cannot mange his share of his own kids’ care and supervision, he must sort it out with the ex, not with you.
(Were you providing such free babysitting before you became pregnant?
I mean it'd have been a bit of a clue, or red flag if you like.)

When I moved in with dh we agreed neither of us was a babysitter for the others kids and I also said I was never cleaning sd room nor making dinners etc if my kids were at their dads. Same went the other way for him. I did her ironing for a while but stopped after a while when I realised it was being put straight back in the dirty wash bin.
In ten years I think looked after her maybe 3 times. His kid his responsibility. He looked after mine once.
say no more and no more family females over whilst your working - you aren’t the paid nanny he is massively taking the piss

Find somewhere else to work for the day and leave, with your child, before he leaves. Up to him to sort. Ignore the tantrum and remind him that you warned him.

DP is a workaholic

He needs to not be with 4 kids, three of which he needs to parent alone when he has them!

You aren't asking for anything unreasonable here,

So basically, the children would rather be at home with their mum, their mum would rather them at home than them be unhappy, but your DH is refusing to let them be at home because it means he'll have to pay more child support.

Then he's gone and brought another baby into the world and not even married you, leaving you potentially financially vulnerable.

No wonder his wife left him, he's certainly no catch.

He needs to organise himself around his ex wife and their kids … perhaps you can do a little, but it’s down to them

Book yourself into a different workspace for the week and sweetly explain it’ll be easier for him as the kids won’t have to keep the noise down for you.

Wondering why you chose to have another child with this man who so clearly shirks the responsibility of his first offspring.

An apology without a change in behaviour is just manipulation @Balaciette and that’s what he’s doing. He’s manipulating you.

I wouldn’t be having it. He could get to fuck if he did it a second time.

But then I’m hard as nails when he comes to this stuff coz my ex took the utter piss.

you wouldn’t say this if genders were reversed

All OP needs to do is literally nothing. Step back. Let him get on with it and the problem will sort itself out.

Don't be silly, she can't do nothing.
He'll go to work, the kids will arrive and she'll be stuck with them (trying to wfh).

She has to do something. Many PPs have advised arranging to work somewhere else for that week. It's hard, and will probably cause "ructions" , but I don't see another choice tbh.

Well if he can't take the time off, he can't take the time off. Plenty of parents can't get time off during school holidays. They have to make alternative arrangements. But to expect you to take time off to look after his children is not on.

But look, if he ends up having to pay for childcare during the holidays then ultimately that comes out of your overall household budget so it's a case of pick your poison. If you don't want to use up your holiday on looking after his children then you shouldn't have to. But accept that it's going to cost you one way or another.

OP how does responsibility for your joint DCs childcare fall, is it 50/50 with drop offs and admin?

Op you are absolutely spot on with everything, funny how when someone is being taken for a mug/unpaid nanny the threads are full of his dc his problem....yet when people like you say exactly that, you are unreasonable 🙄

Is he a moderate/high earner? Can he/you afford for him to take unpaid/parental leave so he can have them when hes supposed too?

How far away is mum? If they point blank refuse holiday clubs can he pick them up/drop them off around his work??

Why did you marry a man who already had 3 children? And then have more children.

Unless he was extremely wealthy and could pay for all childcare or so wealthy he doesn't need to work it was daft. I feel sorry for the children - not you.

If he doesn't let you know his plans (or lack of), I'd make sure you;re not around. The first day the SC are there and he's expecting either you to look after them yourself or Aunty Mabel to come round, get out the house early with your laptop (or whatever you need to work), and don't come back till your work day is over.

It's not the SC fault their dad is a bit crap, but it's not like you'd be abandoning them, they'd be with one of their parents!

One, the OP is working too. She has to work just like he does. Two, he doesn't deign to ask her to take annual leave in advance. Do you think it would kill him to take on some of the organisational load for his own children?

"I am not around over half term. I hope you've sorted childcare out as I won't be working from home that week."

Be overt about it, @Balaciette . Right upfront and in his face NOW that you aren't available. And head out to a coffee shop first thing on half term Monday morning for a while.

You know the old chestnut -
Poor planning on his part doesn't constitute an emergency on yours!

I would ask once more what his plan is.

Then I would plan with a local family member or friend if possible to work from their house for the days he should have the kids that week and plan to be out for the hours your kid is in nursery.

I wouldn't bother telling him as he will use it as a means to argue, just up and leave with the little one in the morning.

He will shout of course he will but it will MAKE him listen to you as he has to deal with the consequences of not doing so. I'd assume you would only need to do this once for him to be forced into having an actual conversation with you that involves him taking responsibility for his children, but I would be prepared to be out for the week this half term and see where that takes you.

I don't think you can out and out say the kids must come less - it is his home too. But i do think you can refuse to be the default babysitter especially whilst you are working too. If you work on the kitchen table I think it is disruptive and therefore you should be the one moving to accommodate the kids but if you work in a spare room with the door shut then I'd do just that in future 'pretend I'm not here' mode. But you need to remove yourself this half term to ensure that he takes this seriously.

OP how would things look if you didn’t WFH? Have you considered getting an office or shared workspace some where? If only for a year or so to break this habit he’s got into?

A (less costly!) alternative may just be to say “No, have an offsite.” and go off on the days he tries to dump on you at short notice. Then go work at the library/in a cafe if reasonable for your line of work. Or see if any of your family will let you work at theirs for a few days. If he questions the offsite just say you’ve booked to work elsewhere over the holiday to avoid having your time disrupted. And go.