I would agree with previous answers. The train from Disney to Paris, visit
http://www.ratp.info/informer/anglais/paris_visite.php and you will see that a day ticket for adult is €18 and child €9. These tickets allow you to get on and off any train, bus and even the escelator to Sacre Coeur (if you are so inclined...) for the whole day. Get yourself a map of the metro and mark off your stops with sights...ie. Eiffel Tower, Champs Elysees, Opera, Louvre etc....
The open top bus is great also, as you could do the tour completely and on the second round get off at the sights that interest you mostly but you will have seen them all even if from a distance. Best advice would be to get the train as early as possible, and the 'red' bus at The Arc de Triomphe (Charles de Gaulle Etoille, is metro stop), they are plactically in front of the Arc, top of Champs Elysees.
If you get that far there is the most fab deli practically across the road, can't think of the name, but it is a glass building with a good restaurant on ground floor. The deli is great, call in there and get your breakfast/lunch/ anything and take it on your open top tour (picnic over paris) and reasonable.... for all the family.
Enjoy your trip. I always do. A fab break even with little ones, and don't forget LeVallee for the shopping.... a free shuttle bus from disney.... take the one going to
, check the times and enjoy too. The kids will enjoy, lots of restaurants there (check out 'Paul'... fab food also).