You are invariably better off avoiding the official tours especially with a youngster. A) because you can do it cheaper on your own. B) Because on your own you can stop and go back whenever you want, C) Because most tours are geared for adults and include adult things.
I will give you a couple of DIY things though. Go to Rome on the train and do the fountain tour on your own, The Spanish Steps, Trevi and Piazza Navona then get the bus to the Coliseum and go back. Or head for the Tiber and do a river trip on a river bus. Barcelona, Get the shuttle into Las Ramblas and enjoy walking up that then go to the beach. Lisbon Catch the train into Cais do Sodre the change onto the underground and go to Alameda then change for Oriente then go to the Parque das Nacoes (Park of Nations ) which is the 98 Expo site. There they have a super Aquarium, Action Halls with loads of things to do for children, a funicular railway and loads more, I can't remember the entrance cost but it wasn't much and it's a super place. Gibraltar I would suggest that you get a taxi to take you to the caves, the apes and the fortifications all of which is worth seeing and then come down to the main street by cable car or the taxi and have a well earned pint. Cheers. .... Neil.
What a great answer - thanks Neil you've been a real help!