Global Mom,
As a family of 5 that just did the Viva Italia, I can totally understand the heartburn over the price. My fantasy was to do a Knights and Lights Tour back to back with the Viva Italia. That fantasy lasted as long as it took me to do the math and gasp. So, we did a week on our own in Paris ( London got nixed) and then flew to Rome for the tour. I highly recommend the ABD tour and truly felt it was worth the price. I also highly recommend doing a trip on your own, as that can be cheaper and a wonderful experience in a different way.
Without speaking a word of French, I rented an apartment near the Louvre-way cheaper than two hotel rooms. We had a huge livingroom and small kitchen. This allowed us total flexibility and we saved even more money by cooking dinner several nights. We spent an entire week, were busy every day, and still did not have time for Versailles or Disney Paris.
We went to the Louvre and climbed the Eiffel Tower and many more of the "Must Dos". We also waited in lines for each of those things. And throughout the day we spent time interpreting Metro maps and consulting guide books. Although our visit to see Mona Lisa was fine, I am sure my kids would have gotten a kick out of the "treasure hunt" activity that ABD includes. I know we got way more out of the Rome, Florence and Venice because of the information provided by the local guides. Your kids are younger, will they be interested in listening to a guide talk about art and history?
The impressive aspect of the ABD tour was the number of activities they cram into a short time. There is no way we could have seen everything in Rome in two days if we were on our own. The day is organized to cover as much as possible. We didn't wait in lines to buy tickets, we never had to search out bathrooms or snacks/water. Meals were ready when we got to the restaurants, we were pre-checked in at the hotels. No need to consult maps or guidebooks either. It may not seem like much, but had we needed to take care of those little things throughout each day, we would have needed to spend more time in each location to cover the same territory.
My advice is to think about how much time you have to spend. If you need to do as much as possible in a week, then the ABD tour is the way to go. If your schedule is more flexible, you can certainly plan a London to Paris trip yourselves, and have a fantastic trip and probably save a considerable amount of money. Do you like to do research and plan?
If you do decide to book with ABD, you might want to seriously consider flying in a couple of days early. Jet lagged kids and parents is not conducive to happy touring.
Good Luck!