Excellent options with children are:
Cuero y Salado Wildlife Refuge.
You take an old train through the pineapple fields to a beautiful lagoon. There is a modern park building with small displays and public restrooms. You can then take either a motor boat or canoe tour of the lagoon. Bring suncreen, hats and water bottles.The Lodge at Pico Bonito.
The lodge allows day visitors. You can use their pool, hike their trails, climb their lookout towers and have lunch at their restaurant. It's a lovely place to explore and easy with children.El Porvenir.
The beach at El Porvenir is lots of fun. There is a river flowing into the sea and it makes for a freshwater "tubing" area that runs parallel to the sea. There's also a small restaurant for drinks and chips.