If you enjoy the beach I would highly reccomend Mar De Las Pampas. It is an incredibly beautiful small town set in a pine forrest on the ocean. It has great restraunts and shopping. Additionally, Mar De Las Pampas and Carrilo are both incredibly kid friendly. Every place you go to has playgrounds with swings and slides etc. "Miradores Del Bosque" is a great little Spa in MDLP with apartment type rooms (studio or multi bedrooms complete with nicely equipped kitchens), great massage and other treatments. This is an incredibly relaxing getaway only 2 hours south of BA. Additionally, almost the entire drive is on a divided highway which is consideravly safer than driving on non-divided roads where so many fatal accidents happen.
As for Estancias are concerned, there are so many I would suggest Googling "Estancias Buenos Aires" and you'll find an assortment.