As stated by the other posters, your room key is required to gain access to the pools. If you are an on-site hotel guest, pool-hopping is encouraged. But you do have to have your room key to access any of the pools at the onsite hotels.
Does the Hard Rock still give out wrist bands? Last time we rented a cabana there, all swimmers had to stop at the towel building, show a room key and get a wrist band. The person there said it was because they were having a problem with people walking over from the parks thinking they could use the pool.
If your kids need a cooling break, definitely take them to the Curious George play area. My kids used to spend hours in there. Even now that they are older, they love to go in it. But bring bathing suits because they will get wet. There is a huge bucket of water that spills from above, water guns, and lots of water jets to dance around in.
Over at IOA, your kids can have a blast in both Camp Jurassic and the play area in Seuss. Also, have them ride One Fish, Two Fish. It is just like Dumbo but with the added fun of having water jets. If you don't listen to the song, and even if you sometimes do, you will get squirted. A ton of fun and refreshing too on a hot day.