(2-part post):
The quickest way into the city centre is on the RER train (Line B). For directions to the train station, from your arrival terminal, see here:
http://www.aeroportsdeparis.fr/Adp/en-GB/Passa gers/Arrivees/QuitterAeroport/EnTransportEnCom mun/EnTransportsEnCommun.htm
(The journey time is around 45 minutes).
RER Line B will take you to St-Michel Notre-Dame station. It takes only a minute or two to walk to Notre Dame Cathedral, so you might want to take a look at it. (Personally, I think that touring the inside of the cathedral would be very boring for most 11-year-old boys).
St-Michel Notre-Dame station is also on the edge of the Latin Quarter. Within a 5 minute walk from the station you'll find dozens of cheap restaurants, serving 3 course meals from as little as �10. (Just follow the 'Quartier Latin' signs). If you pay a little more than the minimum (say �15-�20) you can find some really great food.
RER Line C also serves St-Michel Notre-Dame station. A short journey (4 stops) will take you to Champs de Mars Tours Eiffel station. Here you'll find the Eiffel Tower. You'll need to allow at least 2 hours if you want to ascend it. (There's always quite a queue at the base of the tower. The first lift only takes you part way up, then you have to queue again for the lift to the top). However, the ascent and view make it well worth it. (Your son would almost certainly find this far more interesting than going round Notre Dame Cathedral).