The simplest op is what most people have said:
Fly into San Jose rent a car. Arenal first for volcanoes, hot springs and zip lines. Look into cloud forest and decide if you want to go, many say you can skip it. It’s close if you decide to.
Then Manuel Antonio for rainforest and animals. Yes it is tourist heavy - but it’s the best way to see the monkeys, animals, plus beach with a big group like that. if you can afford it the best thing is to stay at Tulemar. You’ll have the beach right there, all the animals from the national park right there so you don’t even need to go to the crowded national park (we skipped it and everyone we talked to at the resort who went said that was the best call, you can see it all there!) and they can organize a bunch of adventure things for you from there.
https://www.tulemarresort.com/
This trip above is not an off the beaten path trip. Costa Rica isn’t that. But it’s beautiful and fun for the kids and full of fun adventure. If you want a little more remote and are willing to travel I 100% ageee with the Osa peninsula. My kids were younger so I didn’t do it but it’s supposed to be fantastic.