disagreements over paddling pool

Are the 11 grandchildren used to playing with each other. I would put the pool out if that's what you were planning to do. They aren't all going to want to be in at once and over-boisterous behaviour can happen anywhere anyway

When my kids were small ( not babies) I used to put the paddling pool out the front ( shared grass) and half the neighbourhood would be in it
I d certainly put it out and tell the parent that her kids can have a separate session on their own if she prefers, won’t happen of course

Yes I d divide it under 5 s and over 5 s

Pools and kids - extra safety comes to mind as a parent

So many kids around would make me uneasy at around the youngest kids

To each their own - your choice to put the pool up, her choice not to attend

We put up an above ground pool for the whole summer as we have three grandchildren living with us.
It’s always full of the children and their friends and cousins, if anyone objected, I’d say that was fine, but I wouldn’t spoil the pleasure of the many for the few.

I would get a second paddling pool, for the younger ones (the 2 and 3 year old and the 5 year olds) fill it up with some bubble bath in it and some bath toys it’ll keep them occupied . It’s bad enough with older ones running round at soft play in the toddler areas, let alone in the water. I can understand she’s probably worried about them getting injured. I know my under 5s don’t like being splashed in the face with water and can get quite upset, plus I’d be worried about them getting trampled by the older ones when they get excited and are running around splashing each other. I work in a secondary school and 11 year olds are the worst in the playground, the get so daft when they’re over excited and chasing each other. Someone always gets hurt.

Yes I can see things could get boisterous, our youngest is three, and she isn’t allowed in at all if there’s not an adult around, but they all tend to look out for her.

It feels like there is perhaps a history here of one branch of the family being more boisterous than the other, and perhaps the less boisterous children getting hurt and/or their parent`s concerns about that being dismissed? It sounds like rather than deal with that issue, perhaps because they don't have faith they will be taken seriously (by their sibling, i assume?), they are asking you to help them avoid a tense situation altogether.

Herefornow
Now that scenario I do understand.
Not regarding pools, but something else I said I’d do with them the last Easter holidays, eventually there were so many restrictions that I wish I hadn’t have bothered.
If we’re talking two sisters here, all I can say is Good Luck

its your house. You say to everyone this is what will happen....pool lunch whatever you have planned. Those who want to come are welcome those who have a problem can stay away.

Put the pool out.......don't let one person dictate the pleasure of others.
The children, particularly the younger ones shouldn't be unsupervised anyway so suggest their parents take turns in looking after them, sit back yourself and relax.

The thing with the attitude of 'its your house, put the pool out if you want to', is that you're choosing to cater for one kid over the other. And it doesn't sound like one kid is being demanding and the other is being reasonable. One kid has made an odd sounding request (ie no paddling pool), and the other has been a boor about it.

Does OP feel so strongly about the kids playing in the pool, especially considering the perspective that the younger children may get hurt and this may start a family argument? Alternatively are they just doing what one of their kids wants because they stomp louder and a bunch of people on the internet felt more strongly about her personal freedom (important, for sure!) than the reality of keeping the peace in the family. Id be picking my battles and finding a way to make both of my kids feel welcome at family events. I'd also probably probe a bit more what's really going on here because i doubt it starts and ends with a packing pool.

It sound like a parenting style clash to me. One lot are maybe quite considerate and the other are perhaps a bit "wild".
I remember this as a parent who never asked two (separately lovely) families over together because I was on tenterhooks all day.
My own children played with both (separately) and got on ok... though with the rowdy family I was definitely more watchful - individually great and lively children they were somewhat erratic and scary as a "mob"!
I would not be able to relax. Is there a reason why you have asked them both on the same day? Maybe you can do two days, one each?

To each their own - your choice to put the pool up, her choice not to attend

Agree with Hitheres comment above!

I'd also probably probe a bit more what's really going on here because i doubt it starts and ends with a paddling pool.

I also agree with Herefornows comment above. Listen carefully to what is said if you probe

Good luck