AIBU to not pay for them?

Thus. Dh should pay.

@ForRealCat I agree, if my plans changed and my children needed to come to a dinner, I would never expect the host to pay for two extra. I'd explain the changed situation and say that I'll pay for their meals.

A main meal in my local pub starts at £19 now. If they are eating from the adult menu and have two courses this could easily be an extra £60 on the bill? I don't think it is stingy to be unhappy about having an unexpected expense of this level sprung on you.

The half siblings should be invited, but the dad should have instantly offered to pay. It is rude to spend someone else's money for them, without checking what they are comfortable with

Totally agree with this.

OP not been back? Shocker.

I think it would be incredibly mean and stingy not to pay for the step grandchildren. Providing they don’t take the piss and order bottles of Moët, what are we talking another £25? Do you really want to put your DD and DSIL noses out of joint for the sake of £25?

Cancel the meal and just buy your GD a present OP. Problem solved.

Why should she pay a extra 50 or 60 quid? I cannot believe this, it was stated upthread the OP should skimp on something else, what for fucks sake food for herself, not keeping warm? OP may have saved up for this treat.
It has nothing to do with the fact she is the step gran it's to do with changed arrangemts and affordability. It's easy for people with money to virtue signal to say Oh I will pay.
If it was just a gathering round her house of course she would invite them but with the restaurant it is about cost.

I think you've been put in a crappy position OP, I don't think it is right for your son-in-law just to extend the invite without offering to pay. People have budgets, they often need to stick to them, I think it is fine to include the half siblings, indeed I think it is probably better to do the celebration on a day they can come, rather than can't; but it is poor form to just assume the host can absorb the extra cost.

Their Dad should have come to you and said my DC are now home and will be coming so I'd like to split the bill.

I think it depends where you are going and the cost.

Is it a place that has a set kids menu for £8-10 and the kids are young enough to order from it? Then I would just pay the extra, it’s not much.

But if they are teens and likely to cost more like £20+ per head then I think I’d have a quiet word with my daughter - explain that whilst you would love to spent time with the step children, it’s pushed the price of the meal above your budget. I would suggest they all come to your house for a takeaway instead. But hopefully they will offer at this point to pay for the two kids at the restaurant, without you having to awkwardly suggest this.

Unless your going to drip feed that there’s 4 or 5 step children, i think I’d suck it up and pay, assuming there’s just one or two extra!

Either rearrange, or give a lump sum to DD to pay their bit.
For instance if you anticipated the bill being £50 for their part before the other children, then give DD that.

There are polite ways around it.

What better chance to get to know them! Include them, build a relationship. They parents here haven’t tried to trick you into paying for two extra meals. You had an arrangement for a family meal and their other children are now available to attend, through no fault of theirs or their parents. You would definitely be unreasonable to “other” them by rescheduling or saying you wont cover the cost of their meal specifically.

You can feel how you want about it. Buy less for future birthdays to even the deficit. Say you’re delighted, but might need to go halves as you only budgeted for 4 meals. However you want. Just don’t let the children feel weird about it.

It is rude to invite extra people without checking with the one paying or mentioning you will be covering the costs of those you've invited along.
But these are children ( unless theure adults ) I would cover the costs of all the children and let my daughter and her husband cover their meals, I think that's more than fair and inclusive especially if op has a limited budget.

I'd just say, "I won't be able to pay for everyone, sorry, so can we go halves?"

I'd just tell them that you can't cover all 6 as you can't afford it and reschedule for another day.

Honestly OP I would just clarify with your daughter. It may very well be that they don’t expect you to pay for the extra two anyway.

So? Being able to afford six, at a stretch, doesn’t oblige her to.

I’m not sure what your preferred family dynamic has to do with OP, or indeed anyone that isn’t your family. Not all families operate in the same way, or even want or have to.

I'm trying to think myself into OP's son-in-law's head.

Myself, wife and daughter have been invited out by my wife's mother to celebrate my daughter/her granddaughter's birthday. She is expecting it to be a meal for four, and she's kindly paying. Now my two older children, unrelated to her, will be with us that day. Should I expect her to spend 50% more than she was expecting to? Or should I offer to pay for my eldest's meals?

I would be judging him if this were not going through his mind. I'd also be judging OP's daughter if it were not going through her mind too!

There was an excellent suggestion made earlier that OP pay for the children's meals and her daughter and son-in-law pay for their own meals. I think that's the way I would go.

Well you and I can disagree and op can choose whatever advice she wants to follow.

she said she can afford 6 anyway.
and I would treat the family of 5 as a unit, rather than saying those two aren’t my grandchildren so I won’t pay for them.

OP’s choice though

How would you feel if you heard his parents had excluded your grandchildren from meal costs ? For the sake of £50 I’d be happy to keep everyone happy

Depends on the age of these older children. My 13 yo eats an adult meal. This could easily mean £20-£25 extra as there’d be drinks on top too. For two older kids it could be £50-£60 extra on top.

No, OP ‘should’ decide for herself what she wants to do regarding her own money. She budgeted for four, not six. It would be incredibly rude and presumptuous to expect her to pay.

His kids are lesser to her. Considering they’re not her grandchildren, and he has seemingly never expected her to consider them her grandchildren, it’s a leap to claim that he even has a problem with this.

Depends if you think the amount of money you will save is worth the aggravation, or if you just want to make a point.

I would be so surprised if your daughter and SIL still expect you to pay the whole bill now.