Unacceptable

My son always messaged me to make sure I was ok for collecting.

Could you arrange it the other way round? I.E. you tell her you won't fetch him on Mondays unless specifically arranged instead of automatically going. Who rings whom on the Monday each week is up to you, but I suspect you will be calling her.

I am suspicious that her first words were "are you at the school" However, I don't think it is worth falling out over - perhaps for the next couple of weeks you could phone on Sunday and ask if GS will be in school on Monday (just to make a point!)

These things happen. It's easily done. My daughter once forgot to collect my GSs Christmas presents from the post office in time for Christmas day.
No point in worrying about it.

Oh I've had that so many times over my years as a childminder! Very annoying for you to have a wasted journey.
If you "forgot" to be there one Monday they'd no doubt be horrified!
I always charge my normal fee in these instances but I don't imagine that's helpful for you ?

Probably in the excitement of going away for the weekend it slipped her mind that Monday was your school run day. She’s aware now of the slip up and probably feels bad. I’d let it go.

I’d be annoyed to be honest that she didn’t think to tell me that I wouldn’t be needed on the Monday, but I’m sure it was an oversight these things can happen, let’s hope it doesn’t happen again

We are in Australia, so no more regulations here. We are free to travel in our State.

Where had your daughter and family gone for the weekend, or is this another area where covid regulations don't seem to apply?

As a one off, forgivable but if it happens twice then you’ll have to have words (and mine wouldn’t be nice ones)