I can absolutely see both sides.
I think they’re being a bit cheeky asking for the day off – everyone else is doing company related activities.
That said I can see why they’d be a bit narked if for personal reasons (duties or prior engagements rather than hating Brian from accounts, I assume) they can’t make what sounds like a nice weekend away and they feel they’re not getting rewarded like everyone else.
So I think you’re absolutely within your rights to just tell them to be in, but if you decide to let them have Monday off, make it absolutely clear it’s a reward in lieu of the weekend away not any sort of entitlement. It will make them happier, just like the weekend away will make everyone else happier.