Playa Flamingo is absolutely beautiful. Playa Grande is also a cool surf town. I'd avoid Tamarindo with kids. You can rent a house or condo on VRBO. I've also heard good things from one of my coworkers about the Four Seasons in CR. If you stay in the Guanacaste province, I recommend this tour:
http://www.tripadvisor.com/Attraction_Review-g321538-d3263565-Reviews-Vandara_Hot_Springs_Adventure-Rincon_de_La_Vieja_Province_of_Guanacaste.html
We've also done a more adventurous trip to the Osa peninsula. It's a totally different experience than staying in the Guanacaste area. It's a LOT harder to get to, but you see so much more of the rainforest/animals/etc.