We’ve hurt the birthday boy’s feelings?

Yes just explain why your doing it to help the mum out so that the other boy can come the party as she’s working late, just say you will arrange another
time for her son to sleepover , he prob feels abit left out maybe.

Because @ThatBlackCat told you what you should have replied, even to the point that she was instructing you to lie about being paid.

And every single person in whatever country has the misfortune to have you as a citizen clearly all make idiotic generalisations.

Both boys are going to the party, after that is nothing to do with her! It’s kind of you to help another mum.
Birthday boy is also having a sleepover - it was mums choice to make it a family one not a friends one.

It was clear to me until that other poster had me thinking that I'd somehow missed a post.

I think it's lovely that you have formed this support network and - to reiterate - the nutty mother is ridiculous and setting herself up for a world of hurt.

Ok so just read your all updates @sidneytweeney and my original assumption was correct, she is a batshit drama queen. It's her wee boy who will bear the brunt of this in his life, I think you did the right thing in your response, not really reactive and still perfectly pleasant. Whoever said the thing about control is correct; this is a person that needs everything to go the way she wants. Again, I feel for her son.

No, she's heard the boys talking about it.

Do people really behave like this? It is absolutely none of the birthday boy mum’s business what any of the guests’ arrangements are when they leave the party. Does she berate other parents every time her son isn’t involved in another friend’s outing? As for uninviting the two boys, that is horrible behaviour to punish them for daring to do anything fun that doesn’t involve her darling, despite the fact he’s already unavailable. I am sorry for you that you have to deal with a grown-up brat like this.

Sorry I haven't RTFT...

It strikes me that someone is sh*t-stirring.

Someone has told her that you're having a load of boys over at your place afterwards as an extension of the birthday party. The "fun sleepover". It could be BB who doesn't want to stay with GP but would rather have a sleepover.

Or she's just batpoop for brains.

Don't refer to it as a sleepover. Refer to it as "Overnight Childcare" if she asks again.

Unfortunately his mums a twat?

I hope you have sought approval from his mum for whatever it is you are planning to give your boy and his mate for their tea just in case her son would’ve preferred what you are having and would therefore naturally feel justifiably crushed on his birthday.
maybe run all future meals past her just in case?
Jesus, Joseph, Mary and the wee donkey what a total nightmare this woman sounds.

The comparable scenario for adults would be one offering to a place to stay to another friend who is travelling some distance to the party. Or two friends sharing accommodation for a wedding of a mutual friend.

That’s beside the point though, the BB’s mother is simply doo-lally and involving herself in other people’s business when she has no reason to.

OP has also said that the boys aren't a group of three friends - there's no reason, as far as I can tell, that the birthday boy would have necessarily ever been invited to sleep over...

It doesn't matter what the birthday boy is doing after his birthday party - you only get to "control" the time that people are at your party, not what happens before or after! What if the circumstances had meant that the nurse's son stayed at OP's the night before the party, and then OP brough both boys? Would that have been deemed acceptable?
The whole thing is insane. The birthday boy is having a brilliant birthday with TWENTY of his friends, and then off to celebrate even more with family. He's not missing out on anything - you don't get automatic invites to everything all your friends do. For all we know, others going to the party may also be having a sleepover afterwards, or getting together, or doing something fun.

I sincerely hope not, but you never know.

I do think the fact that the birthday boy is going to his grandparent’s after the party is a bit of a red herring because OP still would be doing this sleepover if the birthday boy was just going straight home. Unless Op would have kindly invited him along?

Well, unless she expects the OP to relay the message...

It's just dawned on me that nutty Mum will also have contacted the OP's friend. Goodness knows how that went.

As an adult, I would feel a bit sad if my two best friends planned an after party after my birthday party that I wasn’t invited to. I know that they see each other 1 on 1 at other times but straight after my bday id feel a bit miffed.

@Unexpectedlysinglemum: But surely, if you had categorically stated to all of your friends that your birthday party was closing down at 3.00pm as you had somewhere else to go for the rest of the week-end and would be celebrating with other people, you couldn’t expect two (or more) of them not to decide to do something else together after your party? They wouldn’t have been leaving you out, it would have been you leaving them out, in the first place!

Nothing you say or do will change the mum's behaviour, she is definitely after control and likes provoking drama.

Unfortunately, her son is going to learn this the hard way as he grows up. All you can do is to stick to your guns, which you have, and ignore her attempts at provoking you. I would also be honest with your dc, and anyone who asks, without gossiping about her.