Requesting permission to take children out

I sympathise! My late in-laws were totally oblivious to the fact that they needed to clear it with us before taking our children out and about. They would collect them for the day, and when we asked what their plans were, they would say they were going to see what the children wanted to do. Fine, but our worry was, if anything drastic happened, and they didn’t come home, if we didn’t have any idea where they were going, we couldn’t start to look for them!
It came to a head one day when they brought the children (8 and 6) home from a day out. The children bounced in, brandishing French sticks, Brie and little cakes! They had been taken to France (we lived near the Ferry port at the time), totally without our knowledge. Being in the 1970s, this was before all the security restrictions about travel, they didn’t then need parental permission to travel without a parent and as my Father-in-law was an officer at the Port , it was easy for him to arrange it.
Anything could have happened in France. My father in-law was not experienced in driving there and, in fact, was not the world’s best driver at the best of times. When they came home and told us about the trip, we were thrilled they had a good time but furious that we hadn’t been consulted. Later, once the girls were in bed, we had a private word with my in-laws, making it clear they had overstepped the mark, and visits to them were cut down for a while. Also, they weren’t allowed to stay overnight with them for a long time. We weren’t prepared to take the risk.
My parents, in contrast, were stay-at-homes and a visit to the local park or Abbey was as far as they went! When they had the children, after their much anticipated walk, they would play card games and backgammon etc. accompanied by hot chocolate and a biscuit!
Funnily enough, the girls had no preference as to which grandparents they visited. The French trips were no more important to them than the local trips out. I think being with any of their grandparents was such a joy for them that they truly didn’t mind which ones they were with. Thankfully!!

I always ask my daughter before arranging to take my grandchildren anywhere. Even the one that lives with us , she's 13 and her sister is 15. My grandson is 8 and doesn't always want to come along , depending on the activity. I think its a abit odd they asked your daughter first

I loved being staying with my grandparents on my own. My brother and sister had their chance to go on their own too. It's important for children to have individual attention at times.
This should all have been discussed openly though OP. I wonder why they didn't mention it publicly?

I don't think you should worry too much about your children always having to be taken out together. The eldest will always get involved with things before your youngest (school, dance class, Brownies etc).
It may be a bit overwhelming for your eldest to spend a long time alone without their parents though, rather than without their sibling.
I know DH and I would probably cope with looking after one grandchild whilst out and about rather than two, especially as it's the first time without their parents.
Please don't start setting rigid rules with your In Laws. Try and have a friendly chat about it all.

While I think it's fine to take grandchildren out separately from time to time if they are ok with that and if the parents are too, I would always discuss it with their parents first before even mentioning it to the child.

I agree with Madgran77.
Check with parents first.
It is ok to take GC separately too.

check not heck. And apologies for later typos

I would ALWAYS heck with parents first before mentioning something like that to a child

But also I .ight well suggest taking the older child alone whilst telling the younger one that when they are 6 they will have a turn at the same trip too. Havingng both together might feel like too much for your ILs. Also some one on one attention might be rather nice for your 6 year old and her grandparents