Versailles from Paris

Go to the closest FNAC store and buy the tickets you want (many options of how much to see) or use your Museum Pass if you just want to see the chateau. Take the RER (C?, the one running along the south side of the river) to Versailles Rive Gauche, follow the crowds walking to the chateau, skip the ticket line (if you already have them), line up for security. Bikes might be good if you're seeing everything (takes a full day). Hope you love it!

Be aware that the RER C line divides on the west side of Paris. Be sure to get on a train that is going to Versailles-Rive Gauche. Those trains have the nameplate VICK.

Lora, from the train station to the actual palace /grounds of Versailles in only a 5-10 minute walk max, a six year old will be fine . You should invest in tram tickets while at Versailles ,, Marie Antionettes Hamlet is a must see for most kids( the palace not so much it bores most and is too crowded) and it is a LONG walk from palace. Tram tickets are sold when you board the tram just behind the palace.
As noted, get tickets ahead of time ( or risk waiting many hours) and get there early ( by 9 am) as the line for security can build up too.. Also noted, you can use your Museum for admittance also. Train to Versailles , Line C RER ,, look on those free metro maps on back of brochures etc, of just ask for a map at a manned station. Train ticket is 3.5 each way I beleive. You can decide whether or not to rent bikes when you get to Versailles( with a 6 yr old you will not want to bike in town I assume but just on the grounds) . I personally would not bother with a 6 year old. I would also make sure they have a bike his size,, they do have assorted bike sizes,, but you will need to see what is available when you go.. there is no reservation system for bikes there. Bike rental is down by head of canal.

If you are using the Museum Pass, it will cover the Grand Trianon, the Petit Trianon, and the Domaines des Marie Antoinette (which includes her Hamlet). Technically I think the Petit Trianon is also covered in her "Domain". You would only need to worry about extra fees if there was a special fountain show or something of that nature, which I believe is later in the day. The tram is easy enough to use once you are on the grounds, and then you aren't having to keep up with bikes and walk them from place to place if you aren't in the mood to ride. We walked all the way down to the other palaces and then took the tram back. My wife was 4 months pregnant at the time, so she wanted to walk about the same amount of distance I think a 6 year old could handle. The big thing is to break it up. We walked half-way down the Grand Canal and then stopped and bought an ice cream and sat for a bit. Then we continued on to the smaller palaces. Have fun!

Thank you all for your help. It was extremely helpful. We had a great day.

We took the bus to Versailles from our hotel in the 8th. the bus is slow paced, peaceful, and kid friendly. We went late in the day to avoid the tour bus traffic at the palace (per R. Steves -- GOOD advice). The tickets you use on the subway are good for the bus. The bus got us to within one block of the main gate to Versailles. Also remember you can walk around the grounds after the doors to the palace close, so you have more time to tour then the opening/closing hours would suggest.