We will be in Paris from 5/26 till late 5/30. Is it advisable to get the OC for zone 1-2 and buy the one way RER from CDG to the city for 3 adults and the Paris Visite for my 10 year old since they offer a child rate...or is that rate offered with an adult Paris Visite purchase only? Also, we will be flying to Marseille on Ryanair from Beauvais...what are my choices for transport?
Thanks for the help!

Your best deal is indeed the Carte Orange and Paris Visite for the 10-yo. Please note that the single tickets to the airport come in a child version.

Thanks so much for the info Robespierre. Any ideas on transport for Beauvais?

The departure times for the shuttle bus from Porte Maillot to Beauvais is coordinated with Ryanair's departure times. You must be at Porte Maillot 3hr15min before your flight is scheduled to leave. You can buy bus tickets online in advance or from the bus driver. For details go here: http://tinyurl.com/d3p7s.

Make that "The departure times ARE coordinated...."

Thanks TimS! Will check out the website.