The only lowlights about going with a tour is that we saw a lot of different places, but didn't spend very much time at any one place. For example, we only had two hours at Pompeii, and that entire time we had a guided tour. There was so much at Pompeii that we didn't get to see because we were rushed through it and back onto the bus.
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It is for this reason that we have never gone with a tour group. We plan our own trips and then get to spend as much time as we want in each place. We can go at our own pace. For us this is much more relaxing. We sometimes take guided walking tours and did a one day tour of Normandy but overall we guide ourselves.
We have used Rick Steves' books in our planning and he does have a tour company. His niche is affordable travel so you might find his tours less pricey. I know that he offers family tours which would be good as far as having others your DD's age. Just something to check in to. Not a reccomendation since I haven't taken one.
Frommers has a pretty helpful message board. You might be able to get some feedback there on tour companies.
Your trip sounds wonderful- Italy is next on our list. Happy planning