Selfish or not? Didn't help his ex out

Exactly. But the OP is a woman so everyone expects that she cancel her plans. And when she doesn't, the child's actual parents are annoyed she didn't. It's such BS.

The parents could have paid for a taxi and sent the boy with his grandparents.

Because he is helping out his wife? The one who is working late. She is the one who was supposed to go. He could have done her a favour rather than expecting the OP to help her out. By the sounds of it she isn’t nice to the OP.

The ex’s parents could have got a taxi paid for by mum and dad.

i don’t see why you should have cancelled your session last minute.

Your title puts the view point from you towards the mother

To me I would have done it for the child,

I would not put am exercise class over my own child for this event so would not do it for a step child

It has nothing to do with the ex to me

I agree their parents should have but at the end of the day work is work. A hobby done 3 times a week versus a one off so a child isn't disappointed? I'd feel awful personally.

I agree with others in that, if she does it once, it'll become an expected behaviour. She'll always have to sacrifice her time, to save theirs.

Yep. I’m a legal assistant - have been for 20 years. Worked in all departments, including for two senior partners. Never been “stuck at work.” Neither has anyone else there. Legal assistants don’t need to work into the evening by necessity. It was a choice.

You’ve got to draw a line somewhere. Did you put yourself first? Yes. Are women, especially step mothers, ever allowed to do that? Apparently not.

In this particular case, because the OP's step-son was on his mother's time (I can't think of the right terminology, but it wasn't one of the OP's husband's nights for looking after his son), so if anyone should have stepped in, it should have been his step-father. I think the main thing is that the OP is being berated by her DH and his ex for not stepping up, but the other step-parent involved isn't.
I agree with the poster above who says the parents should have a shared calendar, but it also sounds as if either the child's mother doesn't want the child's father to know about everything that goes on during the time her son is with her, or that the child's father isn't interested in what happens to his son when he's not with him. Either way, this is not the OP's responsibility, it's down to the boy's parents, who seem to be doing a poor job of co-parenting and then expecting the OP to bail them out.

Because it was all very short notice, I would have only taken him if I didn't have plans. But ou did have plans, so I wouldn't expect you to have taken him. If she was really bothered about it, she would have booked half a day, to ensure she got there on time. She could have paid for a taxi for her parents to take him.If she had told ss dad, perhaps he would have booked it off work? The ex has a partner, he could have taken him? Poor planning all round It's a shame he missed it, but seems to be due to poor planning and last minute thoughts. Why it fell to you, I do not know?!

Why would it be stepdads responsibility, anymore than it’s stepmums?
It sounds as if she shouldn’t have got involved with someone who had children already.

I do get what you’re saying. But really if I know about something important that’s coming up for my children, I do tell their dad.

You don't know they are "biological parents" so I don't know why you've used that term.

I'm kind of on the fence with this one.

On the one hand, I feel really bad for the kid who missed out on his presentation night, especially if he won an award. He missed out on the excitement of hearing his name called out and getting the award presented to him and getting his photo taken etc. Really disappointing to find out after the event. And maybe there were sweets and goodies on the night.

On the other hand, he has two biological parents and they should have prioritized the event. I don't know why his DM had to work late. I'd have more sympathy if she works in a medical setting and couldn't get out on time. I'd have a lot less sympathy if she was just disorganised and didn't leave work on time.

You said your DH didn't know about it and only found out that afternoon. So, he needs to make it his business to know about these things. He needs to give his contact details to the leaders of the sport and explain that he's separated and would like to be added to the Whatsapp group (or whatever means of communication they use) so he knows what's happening. Same with school and anything else his son is involved in.

If it was me, I'd probably have cancelled the gym session but I'd have felt pretty resentful.

Ex-wife doesn’t ‘tell’ your DH about presentations and things? Why should he need to be told? He needs to message the football club and request that they include him in all news updates. Pathetic otherwise.

No doubt but they'd want a family member over no one at all.
Poor kid must feel so unsupported. I couldn't have done it to any child of my family.

The child would probably want their actual parent there over their stepmum. I'm a step mum I'm not some kind of consolation parent.

I agree their parents should have but at the end of the day work is work. A hobby done 3 times a week versus a one off so a child isn't disappointed? I'd feel awful personally.

Exactly!

You're making stuff up. Stepmother isn't any sort of responsibility to a child. And I agree the child "suffered" (they'll get over it) because their TWO parents couldn't sort it out

Poor kid neither of their actual parents could be bothered enough to book time off work in advance

She shouldn't even be asking you. And then to have a go when you're busy? How embarrassing for her. Of course you care about your own kids more. She should be embarrassed. The kid has two parents who are alive and seemingly well. Just like a "nuclear family". She needs to get over herself

Leaving work is not the same as giving up one gym session. It's not always easy or straightforward to leave a job in the middle of something, gym sessions are often cancelled with no hard feelings. Who needs three gym sessions a week anyway? I'm all for 'me time', we all need that and I certainly did but the op could have had that on another occasion and done something else.

I agree that it's outrageous that neither parent bothered to book tie off work in advance, then expected YOU to pick up the slack. And more so that they are blaming you for their lack of planning and THEIR son missing out, because of their laxity.

Fuck 'em.

Poor kid.

No way in hell would I prioritise my regular hobby over a child's presentation/happiness.

Fair point. A previous poster said it cost them £40 for personal trainer, but that does not necessarily mean it cost OP so much.
In any case she may have been able to re-arrange her session, or the cost could have been covered by parents. Not a huge sacrifice to make to enable child to attend their presentation.
If the mother was a legal secretary and had to prepare paperwork e.g. for urgent Court hearing I can see she could be stuck at work.