Portable phone charger is a must. Also like other said, cheap dollar store ponchos, temporary tattoos for the kids and if you have girls buy them a dress at Target for $20 instead of in the park for $60+
I'd also recommend one of these chill pad towels. You soak it in cold water, ring it out and it says cold for a few hours. Really helps keep the kids (and adults) cool. Just wrap it around your neck for 10 minutes.
While you can get a cup of water for free from any quick service counter we usually bring an insulated water bottle which keeps drinks cold for 8 hours even in the Florida sun.