AIBU to not pay for them?

I agree he should offer to pay for them, but op should then say, that’s kind of you but don’t worry I’ll pay.

its not being weaponised as a threat anywya. I’m trying to say that if you treat someone as lesser, or someone’s kids as lesser, you very well might tannish that someone’s view of you and their actions towards you in the future could reflect that.

so again, I caution op not to rearrange or say she won’t pay

Because you’re painting a picture of some dire outcome for OP when you have zero clue as to whether that’s anything other than complete fantasy on your part.

Why would OP be resented? They’re the ones that have changed the plans, so there’s every chance already that they don’t expect her to be out of pocket.

It's a child's meal, FFS. It is not going to break the bank and it is not the poor kid's fault that he is going to be with one parent rather than the other. If you have offered to take the family out, you take the family out. If you feel really uncomfortable forking out an extra tenner for a kid's burger, postpone the evening out, but please don't make yourself look petty and churlish.

Why do you think I want drama? I won’t ever know or have any shits to give. I’m trying to help op not to do something that could cause her to be resented.

Tell your daughter you can't pay for her husband or his kids due to your finances.

Is he working? And still expected to cadge a nice meal for he and his kids you barely know off a senior? That's a lot of money if you're living on a fixed income. SMH and side eye.

Pay for them all or pay for none imo. I like the suggestion of you saying you’ll pay for the children - that way it works out the same for everyone as you just paying for the original 3 and your step grandchildren don’t feel singled out.
Whatever agreement you come to, make sure it’s worked out before hand so you’re not awkwardly saying well I’m not paying for you when the bill comes.

When you’re weaponizing it as a threat, you too will have to bear the natural consequences of that.

Their father can pay for them. Not expecting his MIL to cough up for an extra two guests would show kindness and goodwill on his part.

I think your son in law has got a cheek if he expects you to pay, that was not the arrangement when you booked the meal, and you were quite clear who you were paying for.

Plans have been changed, that is not your problem to deal with, of course these children can come, but you are not made of money, so let the Father pay.

I should think surely that your dd and her dh will say that they will pay for his older dc? If not, I think you should just pay it and skimp on something else if it will leave you a bit short. Anyway, that is what I would do because I would not wish to discuss it.

“I’m afraid I do consider it shit on her shoe.“

Of course two kids she has no real relationship with are ‘less than’ her actual grandchild. Considering that there hasn’t been effort put into developing a relationship like that, and that there was no problem with not inviting the ‘less than’ kids in the first place, I don’t think OP not paying for extra guests is going to result in the drama you want it to.

Pay for them all or don’t go , what you are suggesting is embarrassing and if I was your daughter or SIL and you suggested this to me I would just cancel the whole thing .

I see it the other way - it’s not a threat of suffering, it’s natural consequences of being made to feel shit or other. Paying for them would show goodwill and kindness. And the MIL could then expect the same in return sometime down the line.

It's a little awkward but I have 2 SCs and would never expect my parents to pay for them in a situation like this. I'd like to think their dad (or even your daughter on their family's behalf) will offer to chip in anyway, recognising the bill has increased from what you thought it was.

I would also ask your daughter if she wanted to reorganise for another date given the older children will be around - she may take the opportunity to do this anyway since it will change the vibe given you and the SCs don't know eachother too well.

I think it's fair to say (in advance)

"An extra 2 adults (assuming they are eating adult meals) is going to make quite a difference to the bill. Obviously don't mind at all if they are coming along, but my budget means I'm not going to be able to pick up the bill for everyone now"

and let your dd and dh sort it out.
No need for any drama, and obviously none of the dc need to know anything about the arrangements, but just make sure dd and her dh are aware before you go.

I wouldn't expect to bring two extra mouths to feed to a meal I was being treated to! I think it's unreasonably of your daughter and her husband not to offer to pay half.

How much were you intending to pay, can you say you had a budget in mind of £100 (or whatever you expected the meal for 4 to cost) and you are happy to contribute this amount but anything over would need to be covered by them. They may want to rearrange, be happy to cover the shortfall or find somewhere that does cheaper kids or 2 for 1 etc.

I’m afraid I do consider it shit on her shoe.

it’s a family celebration. Only some of the family (the step kids) are less than the others.

op would be very silly to try and rearrange or to say upfront that she won’t pay for the step kids. For the sake of a few quid, which she can afford, she could make her son in law feel as though his kids are a nuisance. And as I say, reap the consequences of that.

I'm surprised at the replies as I think yanbu. However whenever this has happened in my (I'm DD in this situation) with me and DH fighting with my DM as we want to pay half the now larger bill and her saying don't be silly.

Have they offered? I'd be disappointed in their dad if he thinks it's ok to assume you would be absorbing the new cost.

I'd either suck it up (but judge him privately) or else I'd speak to DD in advance and say "it shouldn't be any problem to change the booking, could we do some form of split on the bill as it's increased now."

Well the cost is a point as OP is paying for it and it's rude to invite additional attendees on someone else's dime.

They don’t all have the same family, and will be treated differently by their respective family members.

OP has had little to do with them, and has never taken on grandparent role. It doesn’t seem like something that’s considered a problem either, considering they arranged it without them in the first place.

OP, I would clarify expectations as to who is paying for what with your daughter. You can let her know you haven’t budgeted to cover two extra people, and it may be that they expected to pay for the extra two.

Not the point 2 kids in the same family should never be treated differently it just breads resentment

Woe betide the person who ends up spontaneously having to include my tank of a 15 yo. He's a gym rat, carries me around the house (for a laugh) and eats A LOT!

OP hasn’t suggested that not paying for them will result in the consequences you seem to be hoping for.

As an aside, if someone’s way of getting what they want is ‘do what I say or I’ll make you suffer’ then they really shouldn’t be pandered to, tbh. You can’t force relationships, and attempting to strong arm them is unlikely to result in anything but failure.

Err budget ?
Older DCs eat off the adult menu and may have expensive tastes. Easily another £60-70 on the bill in a standard restaurant.

How old are the extra children? I presume mid teens at the most if their parents are organising contact.
Some older kids eat as much or more than an adult. It could be quite an extra lump of money to find. If OP had known there would be 3 children she may well have chosen a cheaper place to eat or to do something different, or (as others have said), just offer to pay for the children.