Hi Jerry,
CDG to Paris by train: just but "aller-retour" tickets (2 one way tickets) for each of you from the RATP/SNCF ticket office (in either Terminal 1 train station or Terminal 2 train station). Discounts for kids 10 & under exist so if one of your is, do mention that to the ticket seller their age.
These tickets will get you right into Paris from CDG via the RER B line. The closest station to Georges Pompidou centre would be Chatelet Les Halles and from there it's about a ten minute walk from the closest exit. Unfortunately this station is absolutely enormous and confusing so to keep it simple find Rue Rambateau exit on the
blue Sortie (exit) signs. Follow this through the multiple gates you'll need to pass through and multiple sets of escalators. Finally when you exit onto a street (should be a pedestrian street) you'll see a massive church (St. Eustache) which will be your point of reference. Walk away from the church on this pedestrian street (which would be East, you should pass a Bodom store) until you reach Centre Georges Pompidou, which will be a good 10-15 minute walk from this point.
For the
Paris to Versailles train trip, simply buy aller-retour tickets from any Metro or RER station ticket window by asking for "Versailles, aller-retour". These tickets will be good for any transfers you'll have to make to get to Versailles & back, on both RER & Metro.
If you want to take the Metro at all, I'd buy a booklet of ten
Paris Metro tickets (Ticket t+), called a "carnet" and use this the few Metro or bus trips you're likely to take.
Best regards,
Ben