There is a regular city bus that does the same route as all the big hop on hop off busses. . You can sit on it and ride the whole route for a lot less and actually have true Parisians on the bus instead of a bunch of tourists.
There is also a train that will go right from the airport into the city.
Those are your least expensive options, and are just as good as the more expensive tourist choices (unless you have a LOT of bags and really need assistance)
A two day pass is 20 Euro per adult and like 10 Euro per kid (I am assuming 2 adults 2 children) so, 60 euro gets you transport to and from the airport, a two full days of using the transport as much as you like to site see, get quickly around the city, the Montmartre funicular if you decide to go there, and some discounts on some sites. A one way taxi to/from CDG will cost at least 60 Euro so this is a HUGE savings when you do the math.
http://booking.parisinfo.com/z6200e2x26827m466g2603_uk-tour-ticket-paris-visit-pass-transport.aspx
If you were staying longer and were planning on doing a lot of museums, I might recommend the Paris Museum pass with all the above and access to almost all museums via the fast entrance (no lines to the louvre.. Yippie), but for the length of time you will be there, I would just take it slow and do some sight seeing, enjoy some time at a café, and maybe just explore. I WOULD NOT go up the eiffle tower , as the lines are crazy unless you prebook, and the pick pockets are all over. Instead, go to the Tour Montpranas and pay to go up it, no lines, a view of the eiffle tower (instead of the montpranas tour), and the city, and no pickpockets...
