Nobody's at anybody's 'beck and call'... It's a one-off, special occasion with unusual circs (overseas). Pretty much the exact opposite to a 'beck and call' request
I don't agree.
My parents said they had the theatre booked on the saturday night but if we were stuck could try & rearrange or cancel
So they did offer at that point to rearrange or cancel.
DH's parents were happy to come & stay so we accepted the invite & booked our flights
So the OP told her parents they didn't need to rearrange their plans. Now with 4 weeks to go she wants them to rearrange or cancel.
They might have a very good reason. It's a play - it might not be possible to rearrange and get tickets for another performance. They might be meeting up with other people they hardly ever see. Alternatively, they might be thinking "well, we offered, but at this late stage, we can't just drop everything at their beck and call". Or maybe "Well, there are two of them, they don't both need to go".
Also:
my folks have had them for an overnight stay a few times & (seem to) love having them although are totally knackered at the end of it!
Well if that's the case I think OP has a bit of a cheek asking them anyway. It certainly might explain their initial reluctance, and now their unwillingness to change their plans at this late stage.
Is it really that shocking that occasionally, parents have to miss out on things they would like to do? That's life when you have children, isn't it?
I wonder how many weekends away on their own they enjoyed when their children were young!