3 weeks: London-Prague?-Vienna-Krakow?-Porto-Madrid Is this too much?

As a traveler that has been traveling the World since 1981, I found that when I was younger, I was more willing to do many stops with short stays. There was a move way back when entitled "If it's Tuesday we must be in Belgium." As I travel more, I find that I don't wish to make dramatic changes in my travel plans, like flying within Europe a lot.

Not sure if you have been to Europe before or is this a first trip.

My gut tells me that you are visiting too many places not geographically close. You are visiting three major areas, London, central Europe and Iberia.

All three are wonderful, but have you visited Spain and Portugal before, places like Seville, Cordoba, Valencia, Barcelona, Granada, Lisbon, Evora, Cascais, Sintra and the Douro Valley. You could do three weeks just in Spain.

Also, have you done the British countryside? It is amazing, Bath, Winchester, South Wales, Stratford upon Avon, York (especially that city) and more.

Apparently, your plans Will include London, Porto and Madrid. Why not stick to those areas?

I was surprised at how much I loved Vienna, so I vote for London-Vienna-Porto-Madrid. I assume the 3 weeks(21 days) includes flights to Europe and back and you will have 3 other travel days so 16 days divided by 4 is 4 days in each place but I wouldn't even think of adding another city/country.

One of the biggest mistakes people make when first planning a trip is seeing too much. I agree with geovagriffith, stick to the areas where your family and friends are. Assume you will return, especially your 16-year-old.
You need a minimum of six nights in London and that does not include time spent with family and friends. Buy tickets for the Churchill War Rooms before leaving home. Bath also makes a great day trip from London.
I also suggest visiting Lisbon while in PT. Sintra and Évora are good day trip options from Lisbon. You can also include Tavira in the Algarve which is a direct 2h bus ride to Sevilla, Spain.
Vienna deserves five nights if you include a day trip to Bratislava. Vienna is very close to Budapest so plan on visiting central Europe another time.

We now travel in one geographical area but that wasn’t the case when we were in our twenties. I think the contrasts between London, Porto, and Vienna would be quite amazing. I think adding Krakow would be a bit much. These cities are so far apart that the only reasonable way to travel is to fly which is time consuming in itself.

I don’t think you need six nights for a first visit to London, although there certainly is enough to see for six nights. But I would spend more than three. I assume you will be jet lagged which will affect you. Plus, if you are staying with friends/family you are not likely to be as central for sightseeing.

Agree with most of what has already been said and that is to travel to your must sees and plan to return for those places you miss. What is it that draws your son to Vienna? If it is relating to a film/movie that he saw and wants to visit sites depicted in that movie, perhaps steer him toward a different movie(s) shot in London. He is young and could potentially have another trip that includes Vienna. With THIS plan, Vienna is an outlier and far away distance wise. IMHO.

Certainly when getting down to booking flights, look at multi-city options on the airlines’ booking sites. For instance, fly into London and home from Madrid.

Traveling with your child can be a very rewarding and fun experience. Come back to us here on the Forum for more advice after you have your cities set.

Go where your heart takes you. I won't make any assumptions. First determine how much time you really want or need to be with family and friends, in each of London, Porto and Madrid. You might say, I MUST see aunt Gertrude, but 2 nights is all I can take, and other than the obligation Porto is at the bottom of my bucket list. Then come back here and let us know how many nights you have left and you might get different opinions.