For your kids age, I would say it is totally NOT worth it at all. We went when our kids were 4 and 6 and did the beach day. The only beach they let you go to is a public beach that is a pretty good walk, even over the sand. We were panhandled on the beach, and the waves were so very rough they knocked my 4 year old over and about swept her out. We didn't do the slide day because we didn't think it was worth it - and if you don't do the slide day, they don't let you in ANY pools at all. The aquarium is okay, but not great. Your kids won't remember it at all and to be honest, I just don't think it's worth it.
By contrast, I would heartily recommend the Blue Lagoon beach day - we just did that last April when we were on the dream (actually we did the Dolphin encounter/swim and we could go to the beach afterwards.) It was so very easy, completely beautiful and the kids LOVED the beach. They have floating things in the water where the kids can play, and if you want to pay a couple bucks for them to go on this huge inflatable slide, you can - and you know what, they would LOVE it. Dare I say that the kids enjoyed the beach at Blue Lagoon more than they enjoyed
Castaway Cay (heresy, I know!)
If you wanted, you could buy the dolphin encounter for your 4 year old (if you think they would be old enough to like it) and have the 1.5 year old just watch with the other parent.
My advice, after having been to Atlantis with young kids - is to SKIP and really look at Blue Lagoon. It's a 20 minute boat ride, a beautiful beach and everything is centrally located without any panhandlers, and the ocean water is calm like a lake. I would go back there is a heartbeat, with or without kids