I will give an alternate view. We planned a trip with our 13 year old daughter, Amsterdam, Bruges, Paris, London, York area, Ireland, A bit of Cornwall, a bit over 3 weeks. We tried to include her in planning, but by the time we made it to Paris, she wanted to fly home by herself. We made her stick it out, she had a fine time, but traveling with someone of that age takes some doing.
First and foremost, what ever fantasies you might have about traveling to Europe, "What girl would not want to go to Paris", believe me, they just do not feel that way. Do not confuse your wishes with what they may have. You really need to engage them early, find what they might be interested in, and expose them to what they do not know about. You can go on and on about the Louvre, but to them it is just an old building with boring pictures, unless they have context. Great food? Sorry, its not whatever fast food they love.
You really need to figure out what interests them, and be prepared for mundane (for Europe) responses. Just spending time shopping was fine. Going to see a movie that they could see in the US...Fine. Eating fast food, especially American chains? Fine.
Do try to prep them, if going to Paris, have them develop a list of things to do. Try to expose them to art, food, the various sights and see what interests them. Let them come up with part of the itinerary. Basically the more they are involved, the better it will be.