Air BnB CF?

They will be using the shared rooms as well..so by 7 is fair. And spidey sense should be twitching rgds the dodgy excuses that will come into play if there's a shared kitty for expenses

No not unreasonable. Yes CF. Transfer your share of 7 and wait for him to question it. If you don’t do something you will be splitting all bills for the weekend 5 ways and free riding his grown ass kids! They don’t have to pay for themselves but he sure has to!

Sleeps 10, 5 people going, so 5 doubles? Where are grown up kids sleeping? I’d be fuming if they were in the living area and as you’re all friends, the last thing you want is your mate’s kids turning up to ruin the atmosphere (potentially).

OP obviously knows how many bedrooms there are as her DH has the link.

The accommodation costs need to be split per room, rather than per person.

So if the five adults = couple A, single B, single C, single D, and now it's going to be Couple A (room 1), Adult Kids A (room 2), single B (room 3), single C (room 4) single D (room 5) and Family A are paying £240, with B, C D paying £120 each, then perfectly fair - no CFers.

Equally if the five adults were Couple A (room 1), Couple B (room 2), Young children of B (room 3), Single C (room 4), single D (room 5), then it's not unreasonable for Couple A to also bring their adult children FOC to use room 6 or the sofa bed, as each family will be paying £120 per room used by their family.

I can't help wondering why OP won't tell us who is going (adults AND children) and how many rooms they are using v how many rooms in the Air B&B.

So if it's friends going, why has the organiser decided to bring their adult children along without checking that no-one else minds?

I'd say the "kids" pay or I drop out.

OP seems to have gone on holiday...

If you don't' say anything now, you may find yourself splitting meals out, drinks bills and goodness knows what else 5 ways as well!

Are you and your DH both going and sharing a room?

Because this was clearly the plan all along!! He knew what he was doing…..it’s likely sofa bed for 2 in living room so probably 4 bedrooms, 2 couples with a room, solo adult with a room then adult children the other room!
Just say “Hey be better if all 7 of us split the cost as we’re all adults so makes it £86 each person, if 2 adults have paid already just say myself & DH will give them the £34 each they’re owed back and I’ll send you on the other £102 balance we owe you then towards it”
Dont faff about with asking, YOU tell him what’s happening, just say it roughly the way I said it, I assume you’ve a group chat about this so everyone can read it because nobody is going to disagree with you apart from the cheeky self entitled git who thinks it’s ok to invite his adult kids along at your expense.

This.
If the adult children are on a pull out in the lounge or something it eill still decrease the space/access for everyone. You also said it was per adult so couples aren't paying less for sharing a room etc so reply as above.

How many bedrooms and how many of those going are in couples? Are there spare rooms or are they expecting a room for CF kids and everyone else has to bunk in rather than a room each?

Everyone should pay less, and if the host doesn't want his children to pay, he can pay their share. I'm assuming you'll be expected to cover the kids food too.

@Mothership4two - I'm going to go out on a limb here and think that this person is either a BiL or a FiL or Step Father to the OP and because they want to keep it vague they are simply saying a 'friend' has organised this.
I'm getting this because the OP said that this person has been prickly to them before when they have stood up to them, which is giving me more FiL vibes than BiL vibes.

If it is indeed your FiL or Step Father, feel free to say "Thanks, but no thanks. We're going to pull out of this trip because the circumstances have changed, we are expected to pay for Friends 2 adult children and that doesn't work for us"

Their friend I would presume as OP said it's a group of friends.

Who is the AirBnB CF to you @Jbrown76 ?

I wouldn’t ask him, I’d tell him…
so in group chat I’d simply state

“Now that there are 2 extras going we each only need to pay £85.17 per person”

I would clearly state that prices are pp and you think it’s unfair he’s a) invited more along without asking the group and b) expects the group to fund his kids!

in fact at this point I think i would drop out and say his kids coming along wasn’t agreed and will change the dynamics too much!

"As you have changed the dynamics of the holiday I've decided that at no extra cost I'm bringing my great aunt Mildred along as well"
...and see how they like them apples.

Absolutely this!

What is stopping one of the original 5 from inviting their grown child too?

they cover their adult cildrens share not you just send less and say m not paying for your kids

Open a Splitwise group for the trip and share by 7?

Presumably on that basis you can all invite 2 guests of your own.

Too bad if they are ‘prickly’. Man up and say it’s not fair.

Some Holiday Lets are priced per person. The AirBNB owner may have given a price based on five adults of £120 per person for three nights totalling £600. If the host admits to the extra two adults I suspect the cost will be £840 in total. He's basically not admitting to them and will be using the extra rooms without paying for them.

It sleeps 10, so as long as they are below that number the owner won’t care. They’re not charging per person.