I think the Monteverde National Park is the best option because there are several ways to cut the hike into a walk (bypasses, cutoffs, loops, etc.); you might consider a guide to point out animals you otherwise might not notice.
There are also places in town: World of Insects, the Snake Zoo, the Frog Pond, and the Monteverde Butterfly Garden that kids might like.
http://www.monteverdetravel.com/monteverdecostarica.htm

p.s. Cabinas Pinos is a nice place on the outskirts of town that is reasonably priced, well run, and the hosts are very helpful.
http://www.lospinos.net/