all you have to do is get some cardboard and stand it up all around the pool and when they get bigger you can get one of those dog fences to put around the pool and hold it together with a small bungee cord. When they are really small they will be able to fit right through the holes in the fence. Easy Peasy!This pool was one of the larger hard plastic kiddie pools. I believe it was about 5’ across at the top (a little narrower at the bottom) and the height of the walls was about 10” high. I can measure it in just a bit for an exact size.
Only one of my precious, little ducklings made her way over the wall. I didn’t get to see it though. I was in the process of taking them out of the bathtub and drying them off, one by one, and putting them back in the brooder/pool. Petey, my White Layer (and the eventual Bulldozer duck) was first, so she was the only duckling in the pool. As I was drying off the second one, I heard the literal splat splat splat of duckie feet coming quickly across the bathroom floor. Geez, it was Petey, and she was fast. I learned never to let them be by themselves in the brooder again.
I have since learned that ducks go just about anywhere they wanna go...upstairs, downstairs, you name it. And, if they can ever get their head and neck (below their crop) over the edge of something, their body will follow.
The side walls of the pools definitely do not need to be any shorter or the boogers will be out in a heartbeat, especially if they detect food on the other side.
ETA: 4.5’ across and 12” deep.
I got the cardboard at Sam's club from their pallets and the fencing from Walmart online. I have a great big pool and the fence just fits it. On the smaller pools you can just fold back one of the 8 panels. They are around $30+. Those fences come in very handy too for lots of things like dividing "problem" ducks until they get the minds right.