AIBU to not pay for them?

Reschedule OP.
Yanbu.

@Janella yes that might work in some families, but as you can read from the replies here some people are positively looking for reasons to be offended.
I would say the same thing but differently " Its great that Imogen and Robert are joining us, it will be nice to get to know them better. I had put aside £100 for the meal, but if there are 6 of us I am worried it will cost a lot more than that. Are you ok to pay the extra cost ?"

I don't think it's a big deal to say to your son in law beforehand remember I'm going to treat the four of you, so when the bill comes shall you and I sort it between us or do you want to pay for the whole thing and I'll transfer you the money for the 4/5 of us.
Occasionally in my family my MIL might be paying for some ppl but not all of us because e.g. one son and grandson spent the day decorating for her. No one is offended.

Why should OP be expected to pay for them?

If you arranged a meal, agreed to cover the costs for the guests you'd invited and then those guests invited additional people, would you be happy to pay for them, too?

Vile? Get a grip.

So, let's say you arranged a meal for someone's birthday, agreed to pay for all the guests at the meal but then those guests start inviting more people that you hadn't budgeted for. Is it vile for you to refuse to pay for them as they weren't in the original budget?

I wish people like you would remove the fact that they're children and look at the real picture. OP's son in law has invited additional guests that OP was not prepared for and is expecting her to foot the bill.

Don't you find that cheeky? It's not like the step grandchildren were always in the plans to attend and the OP is singling them to not pay for them. They weren't in the plans! They've been added at a later date and OP had never agreed to pay for them.

The son in law is so cheeky to put OP in this situation. Any decent person would say, "Oh, my children are coming as well, but we'll foot the additional cost," not invite them and expect OP to pay. He's putting her in the position of looking like the bad guy if she says that she can't pay for them, isn't he? Rather than doing the right, decent thing and covering the cost of his children.

This isn't that difficult to comprehend and god, some of you people must be money bags if you can just afford to foot the bill for every guest that happens to show up.

How many extra children and how old? I wouldn't mind if children's menu prices, but would be annoyed if they were picking from the adult menu, as that would really bump up the cost.

Pay no attention OP. In the real world, nobody would ever expect you to pay for them

Thankfull not everyone thinks like you.

That’s the fathers fault

the grandmother has limited funds and asked the people she could afford to take, the father added in extra family members.

the grandparent isn’t taking aunts cousins etc, doesn’t make them feel “ less than”

it’s not a case of all or no one

I would arrange for a day they are not there. No you shouldn't be expected to pay for children who aren't your GC, very cheeky of them.

10 pages and no reply from the OP...

Can you not just have a quiet conversation with your dd and SIL in advance about it?

“I’m really sorry but I can’t afford to pay for that many people - can I just check that SIL is happy to pay for his kids, or I’ll cover all the children and you guys cover yourselves?”

Yes the father should pay for his older children as it will start a precedent if she pays for the step children then it will be expected on all occasions. He is pushing his luck. Give the money to the Mum for the four before the event and it will get you out of the situation. They can decide about any extras they want after that. And I would feel differently if you saw the step children on lots of occasions but the Dad is getting a benefit from you.

How many extra DC are there and how old are they?

A couple of extra kids meals won't be expensive, 4 x hungry teenagers having a three course adults meal will mean a big increase in the bill.

It's not unreasonable to re-arrange for another day if you wanted to be just the four of you.

How many of your DD's stepchildren are there, and what age group are they?

Personally, I'd suggest you, DD and DGChild go minus the Son in law and the older children, possibly on a different date.

How do you know? OP hasn’t even confirmed that her SIL is her grandchild’s father!

Another one-post wonder 🙄

On the off-chance that this is real and not just some attempt to goad posters about blended families, there’s still far too much information missing to make a judgement here. Have OP’s daughter and SIL simply announced they’re bringing the other children, without so much as a by your leave? Or have they asked OP if she’d mind them bringing the older children, and the OP is trying to work out what to say? Are the OP’s daughter and her husband likely to realise that this is a bit of an imposition - in which case they might actually offer to pay towards the meal on the day - or are they CFs who think “Great, now we can take all the kids out and good old mum can foot the bill”? Are the children very young and likely to feel genuinely left out if it’s obvious they’re not wanted, or are they early teens who would probably be doing anything else other than having lunch with their stepmom’s mother, and could barely pick OP out from a line-up?

Yes I’m not sure why it makes a difference. Still no relation to the OP.

@Sapienza Oh, I do apologise. Another one that leaps on others to make themselves feel better eh? I think people actually knew what I meant, but thanks for your input!
Half siblings then, but it really makes no odds, it's still not OPs responsibility to pay for their meal! HTH

Except, wait...

The bottom line in all of these situations is two adults who have fucked up, making everyone help them pick up the pieces for the children caught in the middle. And before anyone comes at me, I'm not ascribing blame to the adults; I'm ascribing responsibility.

Not at all unreasonable.

You basically offered to pay for a £200 meal amd it will now be more.

The options are they pay the extra or you reschedule or take ot off the table.

This is a great post, it makes some people feel uncomfortable because they probably realise that being part of a blended family not only involves them imposing random kids onto people but expecting others to magically have a loving bond for kids they otherwise wouldn’t give two hoots about and would never see again should the relationship end

But maybe that's a lot to the op?

If I were her daughter I would see that and offer to pay for them myself.

Rubbish, it is nothing to do with the children, it was agreed that Gran would pay for daughter, Grandchild and SIL.
SIL and daughter decided to add the sil children, who OP stated she did not mind them coming, but not being rich she is worrying about paying for them. It's not about leaving out the step Grandchildren. What was originally agreed was changed and the payment for the two children should not come out of Grans pocket.
I say the above as someone with step Grandchildren who I buy presents for and who I have been to see.

"Step siblings or not!"

@MoonWoman69, what nonsense you write. You do realise that these are siblings, half-siblings if you must.

This is rubbish.

If you can't afford to pay, that's that.

You didn't offer to take them out - they've been added on.

Just tell them you can't pay for everyone and if they complain, rearrange.

As others have said, you could also pay for the extra kids and not DD & her DH. Either way, same amount of people.