Hello dear friend. I think that you should visit areas such us Limon, Arenal, Guanacaste and Monteverde because these areas are full nature , animals, adventure and much more. Step by step you should visit Monteverde to Birdwatching, Arenal Volcano national park , beaches in Guanacaste area, zoos such Africa Mia and you said Definitely Frogs, well, I visit the world´s largest nocturnal-ambience frog habitat at Veragua Park. Check it out http://veraguarainforest.com/froghabitat.html.
HOPEthis helps !!
bye bye

I think Monteverde is a must visit destination. The Frog Pond, World of Insects, Bat Jungle and Butterfly Garden are all great for kids. I've brought my step-son and nephews there at that age, and they loved it.

You could do the standard Arenal>Monteverde>beach loop

I like Playa Hermosa (Guanacaste) for kids, as well.

Some of the canopy tours will take kids that young.

You should definitely check out the La Paz Waterfall Gardens, and if you can swing the cost, stay a night at the Peace Lodge. It's only an hour from the SJO airport. They have a nocturnal frog exhibit that you can walk through and see all the frogs by flashlight. When we were there, they let you hold the frogs. They also have handfeeding of hummingbirds twice a day, and they'll land right on your hand and drink nectar from a tube. They are five beautiful waterfalls you can hike to on the property. Poas volcano is fairly close by.

The Arenal area is another neat place. Penas Blancas river float and the Cano Negro tour are nice ways to see wildlife. There's also a beautiful waterfall hike, and you can swim at the base in a crystal clear river. The Proyecto Asis animal rescue center is a huge hit with kids. You can get very close to the animals, and we were able to feed them when we went.

Manuel Antonio is a really popular beach area for young families. The beach inside the park is beautiful, and there's lots of wildlife like monkeys.

Agree, La Paz/Peace Lodge, and their sister hotel The Springs, are top notch for families/kids, if they fall w/in your budget.

I agree,, try to spend two nights at the Peace Lodge. Your kids will love it!

Thank you all so much for your suggestions! I am now ready to plan the trip.