If all you really want is London and Paris those are VERY easy to do yourself. You can always take walking tours or bus tours once you get there.
Go to the liabary or book store. Get a copy of Rick Steves Europe Through the Back Door (Yes I know he has detractors but he is EXCELLENT at getting you over those "first time Europe jitters and he even has a big section on travel with kids!)
RickSteves.com also has a big message board as do Fodors.com and some other sights where you can get useful info.
Lots of the tours you will see put you out in the suburbs so that's a strike against them. The Rick Steves tours do stay in the city and are cheaper then Disney (what isn't.. those are the most outrageous priced tours LOL!) but with kids I think the freedom of being able to say "nope we are going to spend several hours at the park" is a better idea then being on a tour. (And there are GREAT parks in both cities that your little one will LOVE! The Princess Diana park in London looks amazing, but since I don't have a kid they won't even let me in

)
London is harder to do "cheap" as the Pound is just strong against the dollar but there are lots of affordable hotels in both cities. You just set your hotel budget and go.
Getting between them is also easy. You just hop on the Eurostar which leaves from the center of London and get off in Paris... not at the airport but in Paris.
I can see NO reason to do the ABD for these two cities. I actually stayed at a hotel with them one time (using my Marriott rewards) and from what I saw it's a good way to go to Europe and NEVER meet anyone who isn't American LOL!