First of all, I'm sorry for your loss.
Pop Century Resort is a lot of fun and was recently renovated so all of the rooms are newly remodeled! If you plan on staying at the parks all of or the majority of the day each day I would use the extra day as a pool/shopping/relaxing day, but if you only plan on being half-ish day then I'd use it as another park day. And the reiterate what the PP said, don't try to do everything, you won't be able to. You and your kids should make a list of the top few things you want to do/see in each park and focus on them first, then with any time you still have do other things, that way you can get what you want done and everything else can feel like a bonus! And to add onto what the PP said about Memory Maker, if it's not in your budget then the Photopass Photographers will take your picture with your camera/phone as well, so you can still get great pictures!
We really love 50s Prime Time, the food is good and the atmosphere is fun and unique. Hope you can experience it!