WhatsApp save the date is absolutely a thing here, too.
It means if your dc has a birthday coming up then you know 'oh it's already Maya's party on the 30th, which would have been my first choice of date - I'll have my dc's party the following week then so they don't clash/talk to Maya's parents to see if they'd want to arrange a joint do as they are such good friends/carry on regardless as Maya is having all girls & my ds will be happy just to ask the boys' etc etc.
OP I think I'd talk this one through with your dd - explain that you've already invited the whole class so it would be unkind to leave out 3 - she presumably would be sad to be on the receiving end of this? Explain that it would be rude & mean.
So the options then are: carry on & invite the lot OR scale it back so you invite no more than half the class.
I really doubt anyone would mind - quick follow up message on the WhatsApp group saying 'sorry everyone - we are having a smaller group than originally planned - realised I'd got rather carried away & chosen activity would be impractical with a whole class! I'm an idiot; half term brain! Sorry again for spamming the group, will set up a separate group'.
The kids in the class will have forgotten all about the party over half term, if they aren't particular friends of dd's. & if anyone does say anything to her about it, she can just blame you 'I wanted to invite the whole class but mum wouldn't let me'.
The parents won't, I imagine, give it another thought beyond a) mild relief that they don't have to buy a gift & ferry their dc, or possibly b) mild irritation because they'd pencilled it in & avoided making other plans...no big deal. It's a fun afternoon in the woods, not a wedding in Maui
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