Just to add to Ken's info - the gardens are included in the pass except on the days there is a fountain show or musical gardens. Fountain shows are on the weekends from about April to October. They and musical gardens are also on some Tuesdays. If you're there on one of those days, those parts of the gardens will need a separate ticket and Versailles will be more crowded. My guess is that Ken was there on a Tuesday.
Taking the RER C is usually preferred, but it may not be the most convenient depending on where you are staying in Paris.
The official website has lots of information in English. Start here. Many people like the Petit Trianon and the hamlet as much as the chateau. There's a lot of walking to get there and back, especially with a child. Take advantage of the train - it will be worth the cost.