Is it possible that the older cats are territorial about the litter box and the kittens feel threatened? I'd put another smaller litter box under the stairs or in another location where the younger kitties could have privacy. If that fails I'd have the kittens checked out by your vet for urinary infections, and request a course of broad spectrum antibiotics for the kittens regardless of the diagnosis. Good luck!

First thing is to figure out who the culprit is and take him/her/them to the vet. Many times urinating outside litter box is a medical problem and without addressing it you're always going to have the problem.

In the meantime, get more litter boxes, as suggested. It isn't normal for cats to eliminate in the same place as other cats, although some cats are more adamant about this rule than others. Behaviorists recommend having at least 1 more litter box than you have cats in a multi-cat household.

The other thing you could try is to block off the area where they are going, or to confine them with very little extra space besides the pool you are using now. This forces them to behave properly. Then you can increase the space they are allowed to have very slowly. As soon as someone messes up, go back to the last confinement zone until everyone cooperates again.

The last 2 things will only work if everyone is healthy. A sick cat won't pee in the box until whatever is bothering it is fixed.

I don't envy your predicament right now. I have only 1 cat and she is plenty for me!

I solved the problem. I put a second small rectangular litter box next to the pool and sprinkled cinnamon on the area under the stairs and no problems for the last week. Problem is now, most of them seem to want to use the little box which means it is real full each morning!

They just wanted a change of scenery, I guess. We are having a heat wave right now (nights in the 30's) so once the snow is melted, they will stay outdoors until the cold comes again.

Above ideas were great but there is no way I could have 7 litter boxes in the space available.

Glad you found a solution. It might have been that the higher sides on the pool are something the cats did not like, esp the smaller cats.

The kittens are as big as the mom now and the sides don't matter since the sides of the little box are the same height. Cats are strange!

I agree - cats are strange critters with their quirky behaviors!