Me and my now 6 year olds favorite pools (he was 5 then) are the newly stretched Enchantment of the Seas. It has a great pool area since its renovation with a splash spot, kids pool and adult pool all together. I wish they had itinerarys that were longer than 5 days for this ship.
Our other favorite kids pool is on the NCL Spirit. Its like a small waterpark, with waterslides and playground on the back of the ship. The kids even have their own hot tub. The downside is that adults cannot play in the kids pool and use the fun stuff back there. So you can just hang out and watch the kids have fun lol. Which is great if you are a sun worshipper.
The worst pool for kids I have sailed on would be the Carnival Liberty, it is about the size of a postage stamp, however their Camp Carnival can't be beat. NCL Spirit has a great Kids program as well.
One thing to bear in mind is that the slides are only open for a few hours each day on Carnival Ships and I would not book a cruise based on whether or not the ship has a slide.