Need advice for first time family trip to London/Italy

London Walks: http://www.walks.com/popular-walks/harry-potter

We enjoyed it when we went in March.

I would allocate 4 nights to Rome and 2 nights to Venice.

Also, why schedule London as well as Italy. Skip London for another trip later and add 4 nights to your Italy trip. You need more time for all the cities in Italy, especially Rome.

You may be too late but the WB Harry Potter studio tour just outside London isa absolutely amazing. Get on the site now to see if there are any tickets for the dates that you are in town.

Or you may be able to pay more for an organized trip to Harry Potter which includes transportation from London.

You can also hit the 2 part "Harry Potter and the Cursed Child" at the Palace Theater on the West End.
https://harrypottertheplay.nimaxtheatres.com/hpcc/WEBPAGES/EntaWebShow/ShowPerformance.aspx

Assume you are flying into London and out of Venice.

Is this Harry Potter thing a movie or live show?

Live show.

It's a stage play plotted by JKR that acts as a "next generation" sequel to the Potter novels. If you want to see it I would book your ticket NOW as they are still in very great demand.

The Potter exhibition at Watford gets great reviews but also needs to be booked in advance. However, don't bother with a guided tour all the way from London, it is both more expensive and slower than using public transport. Take a non-stop London Northwestern Railway train from Euston station in central London to Watford Junction station from where there are shuttle buses to the Potter studios.

Don't forget the Warner Bros. Tour with the Harry Potter sets: https://www.wbstudiotour.co.uk

If the OP is leaving in just a couple of weeks, they're out of luck getting HP tickets for the studio tour unless they can get an overpriced ticket on a bus day tour. I suspect Cursed Child tickets would be the same.

I checked WB Studio tours are all sold out:(

What is behind the seam???? They have availability

seems we have no luck for WB studios in London, any other recommendations?

regarding London walks, there are many

http://www.walks.com/our-walks/sunday-walks

which ones is good for my kids who love harry potter?

If you're lucky, you might get to see the spiral staircase from Prisoner of Azkaban (stairway to Trelawney's divination classroom) in St. Paul's Cathedral- it's generally roped off to the general public, but the man who gave us our tour tickets guessed correctly that our girls were Harry Potter fans and took us to see it while we waited for the tour. Also, you can see Cecil Court on your own- it's the street which was the inspiration for Diagon Alley.

If you look at the list from the link you provided you’ll see that the Sunday afternoon walks has one called Harry Potter Film Locations. Sounds perfect for your fans.

http://www.walks.com/our-walks/sunday-walks#harry-potter-film-locations-in-the-city

There’s a Platform 9 3/4 at Kings Cross Station. It’s free and we had a blast. There’s also a Harry Potter store.

Platform 9 3/4 is a big favorite with many HP fans. You can also walk across the Millennium Bridge which was seen in the films.

What do you have planned for Florence?

Honestly, no specific plan, I am just studying, trying to figure out, the best to see in a short time. So any feedback in any destination is much appreciated

Trying to think what would be the most fun for kids at that age in Florence. Yes, there's art galore - seeing David is almost a must, but be prepared for crowds and lines.

The Mecato Nuovo in Florence is cool. There's a statue of a pig. You rub his snout. Good luck is yours. (The market there is also nice for browsing.)

Maybe consider a walking tour in one of the Italian cities? There are tours which cater to English speakers of all interests. Food tours are usually a big hit with all ages. But if you have a specific interest in the household after Harry is taken care of in London, you might want to indulge it.

Rome - The Colosseum is fun. Do not pay for photos with the gladiators outside. You can tour the Catacombs or the Capuchin Crypt which was a big hit. Please note that you WILL SEE bones on the Capuchin Crypt tour.