AIBU to ask DH to stop doing this and is there a nice way to say it?!

If he's going to get huffy, regardless of what you say, just say it. Dh I want to spend some 1 to 1 time with dd. She hasn't had enough attention from either of us lately. Stop asking dss if he wants to come with me without checking with me first. If YOU would like to take both of them out then say so and I will take her out with me another time.

Also, when you want it to be just you & dd, I would term it that you and dd are having a girly afternoon.

Next time dh says he's going out (even popping to the shop) say oh hang on, then shout out to dss - your dad is going to xyz do you want to go with him? Dd get your shoes on, you're going out with daddy.
And keep doing it.
See how he likes it.

Just talk to dss

Do you want to come with me for (childish to a 13 year old activity) or see your mates?

Why bother trying to find a polite way? Tell him to stop, the more impolitely the better

Indeed @Rhaidimiddim has it right

"Stop palming your poor son off on me and spend some time being his Dad" should also suffice

It's absolutely fine to just tell him you want to spend some time with the younger child alone. It's not some weird or heinous crime.

Just say exactly what you've said in your original post "I need to talk to you about something. It's difficult to find things that both kids want to do so before you invite DSC out with me can you check with me first and I can tell you whether or not it is something that is appropriate for DSC or if I'm going to have to think of something different to do. Sometimes I make plans with 5yo and then you invite DSC, I have to change plans and 5 yo is left disappointed."

That would annoy the fuck out of me. He obviously can’t be arsed entertaining his son. It’s sad, he should want to spend time with him.

He’s being a CF so you shouldn’t be tip toeing around trying to find a polite way to tell him to stop.

However, if you are afraid to have that conversation then make sure every time the teen joins you you go to a playground, he will soon realise it’s no fun and stop joining.

Ask him outright if he's with you because he wanted a nanny with a fanny.

Is it actually YOU he wants, or free childcare that he can shag and share bills with?

If he does claim it's actually you that he wants, he needs to prove it by stepping up and parenting his own child himself. If that means rearranging his work schedule, like millions of other working parents do, instead of using you, so be it.

@PPullTheBricksDown hear hear

I think exactly how you said in your last paragraph. Sounds perfectly reasonable to me

you can’t control his feelings that’s on him

I'm usually team step child but this is a different situation. The step child doesn't live with the OP and this is not a family day out. This is the OP wanting to spend some quality time with her child, just the two of them, doing something simple like going to the park. The step child is a 13 year old boy, not a five year old girl. The chances of him wanting to go to the park to feed the ducks (or whatever) with just his step-mum and 5 year old female step sibling are beyond slim. Why isn't the father arranging to take his son out one on one, or arranging a proper family day out for everyone if he feels that bloody passionate about it? I'll tell you why - because it's easier to palm the whole responsibility of his child over to his wife.

Being a family doesn't mean the woman having to take all the kids everywhere she goes.

My three kids don't go everywhere the same - sometimes I take one and sometimes the other two depending on what we're doing. Sometimes DH does. Sometimes we all go out together. Ops seems more like Dad doesn't want to have to do the parenting alone

You seem a bit nervous about having an ordinary conversation with your partner.
Just say that you sometimes want to spend time just with DD so please can he check before assuming that DSD is invited on all your outings.

Women's work: create a family with family activities to include everyone

Men's work: none of the above as they're allowed to be busy with their Big Man Job

She wanted time with her daughter for reasons she explained in her OP, if her DH wants her to take his child then he should ask OP first if it's ok.

Respond "Bra Shopping" everytime he asks what you are doing with DD.

I'd be interested to know how often he takes either or both of his kids out by himself, without you, OP @RonnieRustic

So why doesn’t she tell SS she’s on a day out with dd before he thinks he hasn’t got an invite….preempt that

Im guessing SS was around before OP got together with dh, they came as a package and if dd is 5 now it’s not like she’s only just met him!

They are a family and need to act like one

BS.
On the step-parenting board we regularly get reminded that the step-children need one-on-one time with Daddy, and step-mums should facilitate this.

But when a step-mum wants one-on-one time with her own children, she's a selfish cow who will scar these poor kids for life.

If Dad was working, he should have arranged proper childcare for his son, as any working mother would. Not expect someone else who is trying to be a good parent to their child to alter their parenting plans.

And - let's not forget this not- minor issue! - without discussing it with her first!

Then he should organise his time better to be able to give attention to HIS child on their weekend with him. That's not the OP's responsibility.

Honestly, I'd just say "Please could you stop asking DC if he wants to come with me and youngest DC when we go out?"

He's being unreasonable, not you.

I often don’t take both kids out, they’re both mine! It’s perfectly normal to want to see a friend or go to a play park without a 13 year old tagging along. I wouldn’t do the. ‘You need to spend quality time together, darling’ in front of your SS though, but I would always say I was going to softplay or shopping for shoes or something equally boring when you wanted some 1-1 time with your daughter.

Because she wants time with HER child who is 5, she doesn't want to have to entertain a teenager as well. Why aren't you asking why the dad doesn't want HIS son around?

I'm angry on your behalf, on your DD's behalf and on your DSS's behalf.

Your DH is an extremely selfish man.

Agree
Very sad