Bless you for giving your kids this experience. San Pedro, Caye Caulker and every town has a main playground and will be a must see. Nothing special but will give you the local feeling. You the parent will have to give some time to allow your kids to enjoy, it's up to you. I suggest in SanPedro you get a plate lunch a Master Lees ($3bz for half a chicken and rice) and feed the kids at the park.
San Pedro has two main playgrounds. One is downtown, on the water by the food vendors. There are picnic benches on the beach for you to hang out while the kids play. Don't just hang at Ramons and think that is getting close to the locals. These kids, any kids don't realize the barriers between the cultures, you and your kids will be treated well and don't shield your kids from the locals (imo) who all speak english.
North of town at the cut is another park with play equipment out in wading depth water.
Bikes are cool if you can. We took a ride daily to the parks. Don't forget a soccer ball in your luggage (deflated of course). It will attract the locals for a friendly.
Mainland zoo is "cool" for only a hour or so. Plan your day for the kids, mayan ruins may look cool for you, but my nine year old could care less. But since we've been away she has said it was OK.
Day at Caye Caulker is cool. Novelo bus on the mainland is cool.
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