Hello, Deb. Nice to meet you.
In my experience with live animals, especially ducks, they all seem to know if something is horribly wrong to eat. Of course this is not a solid rule; my dog would probably eat enough chocolate to kill him, if given the chance. But the drinking of salt water seems pretty un-doable, simply due to its bad taste.
If you want to be absolutely sure (and safe), the pool needs to have a fence around it.
I assume that your kiddie pool is sitting on the ground. This presents a barrier to entry, since they would have to get over the height of the pool's wall to enter. Your chlorine pool does not have this vertical barrier, right? I can't help but wonder if the kiddie pool would be more inviting to them if it was sunken into the ground. I know--that's a lot of digging. Still, most of us would dig the hole, if it meant making the kiddie pool more inviting. Besides, when your big pool gets a fence around it, you'll have to make some changes to the kiddie pool anyway--like digging it into the ground.