a day at the seaside.

I wouldn’t have been happy with my in-laws taking my children to the coast ( or anywhere else ) that was an hours drive away as they are terrible drivers and I would have feared for their safety.

Could this be a reason in your case ?

Sounds like she’s exercising her control to me. If so, there’s not much you can do.

No is no. I am sure she has her reasons.

If you have them at yours why not bring the seaside to them?
Get a sandpit and a paddling pool, make sandwiches and get an icecream for them.

We're apparently due a heatwave in the next week or two.

Well if she says no then that's it really. Her children, her rules. Not a lot you can do about it really.

Did you post last year after your trip saying your daughter was angry because you took them to the seaside for the day without telling her? If so I can understand why she is concerned about you taking them. Sorry if I have the wrong person.

I used to argue my point all the time with my daughter.
She was just a person; certainly not the boss of me.

Adults can manage these kinds of conversations, I find.

It sounds as if you really got a bit petty here. The time they spend with the other GPs is none of your business.

You need to respect that she is mum. What she says goes. When she said "no", you needed to respect that - not try and argue your point. She has her reasons. She doesn't owe you an explanation.

That is sad that they are deprived of an enjoyable day especially as they enjoyed it last year. Does your daughter give a reason for not allowing you to take them?

Can you ask your daughter why it was ok last year and not this? Did something happen last year which worried her or has she just moved the goal posts? Are you allowed to take your grandchildren a shorter distance, if so start with that.

I think you need to chat with your daughter about what she will allow or not allow you to do. I can hear your disappointment but you also seem confused about her attitude