Family with 3 teenagers in Barcelona and Madrid late spring/early summer

For Madrid
Do the kids like soccer? We attended a Real Madrid game while there October 2022. I believe they are still playing then.
How about a cooking class? They can be fun, especially if you go private.
Check out the theatre scene. It might be fun to see Lion King in Spanish. If they’ve seen the movie the dialogue won’t matter. Or they might have a circus or something playing.
Bullfight? Too much?
If you go for hot chocolate, skip Sin Gines, mentioned here frequently, and try El Riojana, right around the corner. Our food tour guide took us there and it was so delicious. From ousptside it just looks like a bakery, but there is a cafe in the back.

Personally I think those ages are old enough to do almost what a regular adult tour would look like. Customized to one's liking, of course. And we always took our elementary and middle school kids through museums and some churches-not all day everyday-but a balance. For example, in Madrid you can see the Prado's highlights in less than 2 hrs, then enjoy some time in Retiro Park. We also saw the Royal Palace-how many times do you get to go in a REAL palace? Again it only took a few hrs. Then you can go eat churros! Barcelona especially is full of fun, weird Gaudi stuff! Even Sagrada Familia, it's fascinating and teens would enjoy the strange exterior/interior and views from the higher levels. I agree with your beach suggestion for sure.

Food keeps kids/teens happy just like adults, too. And Spain has some delicious offerings. And odd; my kids thought tiny octopus on a stick in Barcelona were awesome. Not me but they loved it. But the Jamon and pastries, perfect!

For the teens, I’d let them research both cities and then let you know what they are interested in. For anything I’d suggest they would probably say “boring.”

There are a number of good churro and chocolate locations. Each is good in its own way, so it comes down to personal preference. Go to one where the churros are being made fresh and haven’t been sitting around. I like San Gines and we’ve also enjoyed Churrería Chocolateria 1902 too.

Thanks for all the helpful suggestions. I will be sure to pass them on.