At 4, 68 and 70 it seems unlikely that your family woul have any interest in attempting the long distance trails thru Corcovado as it can be extremely hot & humid & the shortest of these trails requires fording rivers that would be over the 4 year old's head. That leaves you with limited choices if you want access to the Osa's wildlife.
Rent a house near Cabo Matapalo and hire a guide to explore the surrounding area. Hpu would have to do your own cooking and hiking the Trail from the LaLeona entrance doesn't seem worth the effort since it's a hike just to bet from Carate to the entrance to the park.
Stay right outside Drake Bay where you can access Sirena ranger station by boat. There are a number of hiking trails around the ranger station but the point of a visit there isn't really hiking - it's tracking wildlife and the wildlife there is most abundant at dawn and dusk. The lazy man's method of enjoying all this wildlife is to base ypurself in Drake Bay and arrange a 2 night stay at the Sirena ranger station with a guide. Then you'll see all the wildlife you could want, endure relatively primitive conditions for only q couple of nights and then boat back to Drake Bay.
Consider a rental furthere up the coast in Uvita where there is plenty of wildlife and take daytrips by boat to Corcovado.