If you are in Dominical you can check out Hacienda Baru Private Reserve http://www.haciendabaru.com/ , kids love it my 4 year old and 8 year old both the went on the zip line which is very mellow and had a great time and felt it safe. I am not a big fan of Manuel Antonio but it has some nice beaches but very touristy and the wildlife has been fed by so called guides and visitors so its not really as "wild" as it used to be.
There are day trips to Corcovado by boat from Uvita beach, takes about 1 1/2 hours each way and a full day trip. The boat ride is actualy a lot of fun and you can get to spot whales and dolphines. The hike is more of a walk but its a personal choice, I would have no issues taking my 4 year old on a day trip but deffinitivly not overnight. Though probably a bit tiering. Here are some local tour companies if you decide on Corcovado.
Do visit "Ventanas Beach" (25 min South of Dominical) it has caves you can walk into during low tide, kids love it (supervised) expecially if you tell them there is a pirates treasure in there!
You can also localy visit http://www.rancholamerced.com/ and Oro Verde reserve.
A great idea for lunch in the area would be Talipas Pavon as you get to fish your own Talipias and they cook it (filet it for kids) makes a great impresion on kids as well.