What do you think of this situation? unreasonable or normal?

I'm so sorry your children are being treated like this, OP. No, they're not her children but if she wasn't prepared to love them and treat them with kindness then she shouldn't have married their father. She didn't have to take them to the aquarium but neither did she have to dump them and swan off to a better offer.

My sister is a step mother and if I invited her for a day trip she had better bring her husband's children - my son would be devastated otherwise! We all adore them! I wouldn't say we treat them like our own family - we respect that they have a mother and plenty of biological addticts and uncles and we're not replacing them - but when they're with their dad and my sister it is expected that they come to everything the rest of the children come to. Love grows if you nurture it. There's plenty to go around.

Shame the dad doesnt see himself as part of the package then!

Should have said she'd go with the three children. You take on a man with children, they come as a package.

And their father is married to her, they aren’t. Maybe they would sometimes love to have the chance to be treated as his priority by him, rather than passed on to her.

His children, his responsibility.
He knew it was his weekend.

otherwise he steps back and just goes for every other weekend and increases the money he pays the mother for them.

Silly Sarah getting involved with a man with 2 children.

I would think any 7 or 9 year old would feel left out and jealous. Was it some sort of punishment not taking them? Were they being naughty? Could he not have taken the day off work and did something with them instead?

Absolutely this!
As always a woman is at fault for a poor father.
She wasn't very nice but probably sick of him taking advantage.

This, unless they didn't want to go

No she’s a step-parent whose child is their half sibling. If she has a problem take it up with John when his children are asleep .

Stick up for your kids. I was just thinking I didn't do this for mine and got quite upset about it.

It’s John’s job to protect his children.

It’s a bit like when people say the OW is equally to blame - she isn’t, you are blaming a woman for a husband/father not protecting his family.

Sarah’s actions weren’t nice, but it’s dad’s place to ensure his children’s wellbeing so he shouldn’t have left them with her.

I think two extra children at that age would be fine. I think it was rather mean tbh unless they are badly behaved with you.

But not many mums do, because they actually, you know, like their kids and want to spend time with them?

Op, I thought you must have been their mum and have seen your update. Like another poster has suggested I would say to your ex that the children were upset on Saturday and rather then it happen again can you review the split of visitation so that he is definitely around to spend time with them when he isn’t working.
Hopefully that will be the kick he needs to realise everyone is aware of the impact on the children, if he asks to drop some of his days then if your work/life allows I would not send them rather then the children witness arguing and feeling left out and make sure he ups his maintenance accordingly.
They must have met when your children were tiny, and had a child of their own pretty quickly and it sounds like their Step Mum is getting fed up with looking after extra children in his contact time when he should be there. I couldn’t have done it to the kids in her shoes, they aren’t recent additions….. However is for you Ex to sort so I would definitely mention it to him.

Because lots of shit dads with other women to palm their kids off to prefer that, than having the drudge of school.

Exactly. OP is a CF.

Why are people assuming 40% is every weekend? There's lots of other ways they could be splitting things?

Awful way to treat those children. I don't know how anyone could do that. The issues they're going to get from feeling unwanted. It's just awful.

I reckon this is why OP’s not replying… She knows she won’t get more £ for more custody because her ex is difficult, and the custody schedule is such that she currently has all weekends free to work or have time to herself. Sarah’s unpaid labour has been benefiting both parents.

You need to tell your ex to stop dumping his kids with his partner and actually be their parent.

Sarah is mean. Poor kids being left knowing that they can’t go. It doesn’t sound like Sarah likes the kids or she’s had enough of doing all the childcare on the weekend while John decides that he’d rather be in work.

Assume you wouldn't dump them if you were supposed to be caring for them and got invited to an aquarium. You would take them with you if they wanted to go.

In the light of your update I wondered do you actually need your children to go to their dad's (e.g. does it provide your childcare to work?) if not I would just tell him to let you know when he has time to look after them/see them (his self) as otherwise they won't bother coming.

Your children have two parents and it is 100% the responsibility of you both to provide it.
Not to expect his new partner with her own child to want to take care of 3 children on a day out.

Having been in Sarah’s position, I agree with this

He’s self-employed and having two kids in tow didn’t stop him working; it seems quite likely he decides his working hours.

There could be a dripfeed that they’re broke and paying off debt but in that case, nobody would be going to the aquarium.