Thames river cruise to Greenwich if weather is nice.
Fun musical.
Churros in Barcelona!

For London, I would definitely add Hampton Court (Henry VIII's palace on the Thames). My DDs, 6 and 4 at the time, loved it. We also enjoyed the canal boat tours that ended up in Camden, the British Museum and a visit to the Tate Modern via a walk across the Millennium Bridge.

Barcelona - go to Paco Meralgo for tapas. DELICIOUS!!! Someone local told us about it and we liked it soooo much we returned twice while there. They have a fancy brother that we also tried but it was not as good. I don't even remember the name of the fancy one.

Anyway, here is the address: http://www.restaurantpacomeralgo.com

Dirección
C/ Muntaner, 171 08036 Barcelona

It is not kids friendly per se, BUT we took our 2.5y old without any problems all the 3 times we went (during a 7 days period!). So, your 7 and 11 can go for sure, unless they are spoiled brats who can't behave in a restaurant - what I doubt.

London - didn't read the replies, but I went recently with my 3 y old and we had a blast, even though it was very cold. I am not sure your kids like that but we enjoyed lots of tea (god, that cream is something!) instead of meals, since unfortunately, I didn't find any good restaurants and the food generally sucked. I am sure there are wonderful restaurants in London, we were just unlucky.

I wanted to have tea at the Ritz and I loved the experience. My daughter of course enjoyed the treats, but she didn't really care for the ambience. She didn't pay, so I had nothing to lose. Same with tea at Harrolds.

The best museum for her was the science one - it is great! Just behind the museum of natural history.

Those are great ages for traveling! You and your kids will enjoy both London and Barcelona. We lived in Europe when our kids were the same ages and here are some of our favorite things about each spot:

London:
Princess Diana playground
http://www.royalparks.org.uk/parks/kensington_gardens/diana_playground.cfm
The London Eye (be sure to buy your tickets on line to avoid the long line. Our kids also loved the carousel which was located near the eye....it was a very fast one!)
http://www.londoneye.com/
In they are Potter fans:
http://www.muggletours.co.uk/
Natural History Museum
http://www.nhm.ac.uk/visit-us/whats-on/events/programs/naturelive/the_museum_at_war_%28webcast_live%29.html?date=06.06.2013



Barcelona:
The most amazing magic shop...old, charming, and our kids got to go behind the curtain to learn some tricks that I still can't figure out!
http://www.elreydelamagia.com/en
Papa Bubble (fun to watch them making candy)
http://www.nytimes.com/2006/02/05/travel/05forage.html?_r=0
Joan Miro Museum
http://fundaciomiro-bcn.org/?idioma=2
Circle Dances on Sundays in front of the Cathedral...join in the fun. It really is a unique experience.
http://www.oh-barcelona.com/en/blog/2012/culture/dance-the-sardana-24063
Go to a Barca soccer game or at the very least buy a soccer ball and start a pick up game in a park with local kids

Barcelona- Guell Park- Gaudi designed park that contains some of the most interesting architecture

Monjuic Park and all of the attractions within it especially the castle at the top. It is an old fortress. You take a gondola up to the top. It is very cool and you can see the whole city. The Olympic Stadium is also in Monjuic Park. There are many other atttractions within the park. Just look up Monjuic Park and you can decide which ones your kids will be interested in

Take a walk down Las Ramblas but you will also want to take a walk around the Gothic Quarter towards Via Laietana and on the other side of Via Laietana. I found those streets to be even more interesting than Las Ramblas itself.

The Palau Guell is worth seeing for the incredible architecture. We did a few museums but I was thinking your kids wouldn't be interested.

I'm Jewish so we went to the synagogue they uncovered in 1987 that is now a small educational center. I was interested in hearing what it was like for Jews in Barcelona pre-Inquisition. It is the oldest synagogue in Europe. If you are at all intereted in that it is in the Gothic Quarter and the people that work/volunteer there are very knowledgeable. Most are studing some aspect of Jewish/European history.

Make a resevation at the restaurant Sensi, Carrer Ample 26. The food was some of the best we had and we had some wonderful meals while we were there. We lucked out and were able to get in without a reservation, but we got the last table.

Warning: Most places do not even open until 8:30 pm for dinner. If you go at that hour it will be easier. Have a snack.

My husband and I went for our 10th anniversary last October.

In London planning on going to the Tower of London, changing of the guard, Big Ben, Covent Garden. Other ideas for there and/or Barcelona?