We went on a Europe trip this past summer, with DD23, DD15, and DS12. We did the Tauck Grand European Tour--please check out their itinerary, it's meant as a family tour. The youngest tour member was 8. See if there are ideas you can steal. Tauck may have something similar for Barcelona--I must say, they did a great job of balancing kid activities with more grown-up ones.
In London, we liked the Shard, the Tower of London, we did a bus tour (hop on/hop off--there are several, and you can get ones that include tickets to sights like the Tower). Kids loved the Harry Potter tour. I would have liked a Jack the Ripper tour, but we couldn't fit it in. High tea wasn't my thing, but if you have people who like to be fancy, it would be fun. Borough Market was a fun place to shop and eat. The London Eye was fun, but..I'm afraid of heights. I was doing okay, we were literally the next car to be unloaded, when the wheel stopped...and reversed...and took us all the way back up to the top again. Our guide said he'd never seen that before in 8 years of touring. An optimist would say that we got two rides for the price of one, but I was so, so glad to get back on the ground!
In Paris, we did a Seine river cruise, a chocolate dinner/tasting/kids made chocolate bars. We also visited Montmartre, which was very cool. I was less impressed with the Palace of Vaux le Vicomte. Obviously, the Eiffel Tower and the Louvre.
We stayed in hotel rooms, so I can't help you there. I will say, study maps, etc.--Rick Steves has books on the cities you mentioned--and figure out where you'd like to stay. After hitting the big touristy places, try to find little gems in the area you're staying--parks, small museums, other things that could be interesting. Try the local restaurants, poke in the local shops--London, especially, has shops that have been open for hundreds of years, and some great flea markets.