We don't have a pool, but do have an oversized in ground spa. (Large enough to teach kids to swim in it.)
We thought we did great by putting in a wrought iron gate at one side of the house, closing off the area with the spa. Well, when my daughter was small, she just crawled under it like she saw the cats do. When she was about 3-1/2, she climbed up it...yes, she's part monkey! It's just straight bars, but she can shimmy up it, then she opens the latch and wiggles her body back and forth to open the gate.
Certainly have to find out something else, as we've got twins on the way next spring.
We did have a cover that requires a special tool to get the cover on and off, put it was very difficult and I couldn't do it...there were large springs that held the straps to some metal pegs drilled into the ground. Let me say that my husband had me do the research and that was NOT what I thought would work. And I was right. Anyway, we don't keep a cover on it at this point. With only one child, we can keep an eye on her and know when she's outside. Next year at this time, no way. We will definitely have a new and easier cover on it...probably the first one I suggested! (A friend has one on her full-sized pool. It's a net with squares that are about 3-4" and it can be put on and taken off very easily.)