to make DS (4) go to his swimming lesson on saturday

[Choufleur]

instead to the party of a boy from his class?

He's not even really friendly with this boy and I've paid for the swimming lesson. The party is at 9.30am.

I admit I might BU.

YABU - swimming is presumably supposed to be a hobby/for enjoyment as much as learning to swim. Defeats the object imo if you force it like a chore. It's only one week, he can miss the swimming. DS has to miss around 2 swimming lessons out of every 10. Yes it costs be £12 a pop, but it's for him I'm doing it. I spend the £12 whether he goes or not, after all