I can't speak to you or your children, but there's a LOT to do in and around Napflio. I could easily have filled three full days with no time whatsoever for chilling out. YMMV.

I doubt you will run out of things to do in and around Nafplio. If you do, you can take a bus or taxi to the port at Ermioni for a day trip to the islands of Spetses or Hydra.