In London, my daughter and I really, REALLY enjoyed our "Jack the Ripper Walking Tour". It wasn't scary, but had just the right amount of "creep factor" for my daughter (who was 19 at the time). It takes place shortly before sundown and has a lot of interesting information and "theories" about the JTR murders. We booked that through viator dot com and it wasn't too expensive. We also booked a day tour to visit Stonehenge/Lacock/Bath, also through viator. We paid a bit extra to tour Stonehenge at sunrise and to be able to walk among the stones. Most Stonehenge tours keep you behind a fence that circles the stones, so you're about 30 feet away. Our bus drove through the morning fog and then, POW, there was the stone circle. It was a great atmosphere! A guide was there to tell us about the stones and answer questions, otherwise, we were on our own. It was quite spectacular to watch the sunrise and the fog lift as we walked around. Highly recommended! After time in the gift shop we drove to Lacock. Lacock is a perfectly preserved English town that looks like it did in the 1700's (and earlier). It's the town where scenes from Harry Potter and Pride and Prejudice were filmed (among other movies). We did a tour of the town, stopping at the building used for Hogwarts School, Harry's childhood home, etc. We stopped and had breakfast at the George Inn, which dates to the 1500's. The charming pub houses it's original dog wheel, where special breeds were put to run and turn the meat on a spit over the fire! Out back is a reproduction stockade, perfect for that wacky family photo. In Bath we saw Jane Austin's house and then were turned loose to explore the Roman Baths and the attached interactive museum. Very interesting for all ages! We tasted the terrible Bath water (it has steam coming out of it, lol!) and took lots of fun pictures. We had just enough time for a quick and tasty scone with clotted creme before catching the bus back to London. A great day! Other fun, kid friendly London things: the Hard Rock Cafe (London has the original), the theater, walking around near the London Eye, you can visit Jubilee Gardens there, there's a park, carnival rides, characters walking around, people selling their art or charming snakes or performing illusions- all on the street and bridges. Take a cruise on the Thames, or walk around Buckingham Palace and explore.
In Paris, take the kids to the Catacombs! The underground burial tunnels are fascinating! Just be sure to get there about an hour before opening (opens at 10 I think) to secure your place in line, as the line can get to 2-4 hours long! It's very cheap to get in though, about 10 bucks per person.
Have a great time!