To not want to holiday with this child

I think it would be reasonable to say “we’ve given it some thought and we don’t think it’s right for us. We hope you’ll still come but we’ll completely understand if you want to holiday with your partner instead”.

Saying “we don’t think it’s right for us” keeps the focus on you, not her or her child, and it’s not really a debating point - you get to decide what is right for you.
It might be helpful to him if you send him a neutrally phrased text like that so that he can show it to his girlfriend with minimal drama.

Sometimes, if I want to say no to someone who has asked me a favour, I am deliberately obtuse and go with “you are so kind to offer to come with us/it’s so nice of you to think of us but it won’t work for us on this occasion. Thank you though for the kind thought”. It confuses many a cheeky fucker into stunned silence.

They're not part of OP's family, @Fruitbat99 .

If a woman posted on here that her partner of less than a year was her child's stepfather, she'd be told that it was too soon to introduce him to her child never mind call him a step-father.

Unhealthy how? OP's son wants them to come. I'd see if I could accommodate if it was important to him, without making the holiday all about the child. That's very possible, I've been on holidays like that.

How old are the teens?

Just if they're 15yo plus, even taking SEN out of the question it can be really hard to keep them all happy.
I have 6 years between mine, and they're all happy to compromise, and do the other's things to a certain extent and they're used to each others' fads and likes... but holidays used to be a bit of a flashpoint trying to please them all.

You'd say "we'll do this for #1 on day 1, #2's thing on day 2, #3's thing on day 3". Then find that the oldest decided not to do day 3 as it was something that they'd really be bored. The #2 would decide that they'd like a day to themselves too, then #3 would sulk because they were too young to be left alone and it wasn't fair that the others didn't have to do two things they didn't want to do...
And you lost the family holiday at this point as they weren't wanting to come out all at the same time.

I'd suggest working out what you would be doing with the teens, and explain this.
"We are planning on doing X, Y and Z" if you want to come on them then you are welcome.

Or suggest you do every other day together. Look at what you want to do and what would be more suitable for the younger one, and suggest they join you for those but not the other things. In all honesty, having holidayed with family, I find that better to do anyway, even with cousins who get on very well and are similar ages. The days away from each other means they're far more tolerant on the days they're together, both of their cousins, and of each other.

But as ds is on the other side of the country, I'd go for "we see so little of you, wouldn't it be better if we meet up at yours/at ours so we can dedicate the time to you".

Well some people get pleased and some people get disappointed whatever happens.

My view is that it doesn't have to change the whole holiday, and that making a compromise to allow them to be there might just be good for everyone.

Obviously OP isn't going to invite them, nobody wants them there, but that's a bit sad for DS who clearly sees this as a serious relationship if he's asking about a holiday a year away.

I don't get it. This is nuts surely? It's obvious that a girlfriend of 9 months doesn't get to invite herself and her child with additional needs on someone else's holiday.

The way this should have gone ...

1. The gf has the sensitivity to realise you don't ask to go on someone else's holiday, especially not with a high needs child nobody else knows so she doesn't ask.

1. The adult (yes adult at 23!) child wants the gf and child to come so asks their parents if it's ok, without mentioning to the gf. They are free to say no and nobody gets the hump if they do as this is totally their decision and completely reasonable.

1. The adult child says to parents that he is likely to want his family holiday with gf and child and offers to back out, but this then gives the parents the chance to invite the gf and child IF THEY WANTED TO.

Anything else is pandering to entitlement.

Telling @Birdsting to offer another holiday is crackers. It's ok to have boundaries especially with new and controlling partners!

‘Pleasing’ your son - that sounds very unhealthy, as it is very likely to ruin everyone else’s holiday, but as long as you please then the fall out doesn’t really matter?!

I know. I was clarifying.

No, it isn't. It's up to everyone who is going. Nobody is stopping him going on holiday with his new 'family' separately.

I didnt say it was ableist as I dont believe that. I used ageist to show how stupid it is to accuse OP (or someone similar) of being ableist. Its neither.

Say no

It's your holiday

It's not ageist or ableist - OP isn't against said child having a holiday, just not gatecrashing her existing plans.

I wouldn’t facilitate this. We are a close family but my parents have done holidays with my siblings but not included me as I have kids and it makes it a completely different holiday. I can totally understand it!

Think that's up to him to decide

You said it better than me

And what about everyone else.?

People cannot just expect to be added to an already planned holiday, which likely won't meet their needs, and then complain when it doesn't happen! It's perfectly ok to reject that option!

Its the new in word, thrown around with abandon

Its not about someone having SEN (although for OP it isnt about the difficult behaviour on holiday she doesnt want a child there, perhaps this is ageist instead)

But for me I wouldnt want to be on holiday with someone with difficult behaviour, child or not, SEN or not. Thats my choice. If its my child or relative thats different, I need to deal with it but I get to choose.

I've an autistic child who doesn't have needs as severe as the child in the op and I can't think of anything worse than trying to take them on holiday.

Pleasing my son, who is the one that's requested they be invited, is something I'd be more than happy to do.

People do not have to spend time with people or children that they dont have to or dont want to. Presumably the child may not have such difficult and challenging and exhausting behaviour if he didnt have SEN, so therefore OP might be able to feel able to spend time with him.

People shouldnt feel obliged to do something they dont want to do, for 'fear' of being called names for it.

I think the OP has explained that having any kid there changes the holiday. Even a quiet, go along with the flow, not causing a peep, kind of kid. A child attending = not the holiday she wants. A child with SEN attending = not the holiday she wants. OP and her DH paying for the holiday = their decision.

Why? That’s just plain old people pleasing. It’s supposed to be a holiday!! Relaxing, enjoyable etc.

I think its fair enough to say its not a child centred holiday and on this occasion the answer has to be no.

Why not chill out a bit. Have them come but no need to spend every day together. Meet up for the things that you could do together and split up to cover the trips/activities that aren't appropriate for children.

It's going to look like a rejection of the child otherwise. Don't give the controlling GF a stick to beat you with.