Europe with older teens

Stay north where it is usually much cooler; Scandinavia/UK.

Latitude and altitude can be your friends! Staying north, as Wray mentioned, or visiting higher altitude places in Switzerland and Austria (even northern Italy) could be options.

Unless you've already been there/done that: Scotland, Ireland, Iceland, Norway, Denmark, Finland, Sweden, Estonia, Latvia, Lithuania. Of course, heat waves can hit anyplace, including these (so can rainstorms, for that matter) but statistically these northern countries are a good bet for reasonable weather in midsummer. Avoid anything in southern Europe (save Mediterranean destinations for spring or fall).

Where did you visit on your first trip with the kids? As suggested before England, Scotland and Ireland may fit the bill. I'd also like to suggest Galicia, Asturias or the Basque Country in Spain. Would your kids be interested in walking the last few miles of the Camino de Santiago into Santiago de Compostela? Avoid middle and southern Spain, particularly Andalucía in summer months if you want to avoid the heat. What about someplace in the mountains of Bavaria? Garmisch-Partenkirchen and surrounding area has lots to see and do for young and old alike and is close enough to include a visit to Salzburg or Innsbruck. Poland? Have your kids given you their input?

Thanks for all the suggestions so far. Our kids have been to London Paris southern France Mallorca and Barcelona. They were thinking London (again) and some small town near London , Berlin and Prague. My question is Are these good choices or are we missing things?
Thank you!!

Also: how long is your trip, and when exactly are you going? "Summer" (depending on how you define that) could cover a lot.

While London would certainly be a better bet for cool weather than, say, Rome, if you want to avoid the worst heat that might hit, avoid the biggest cities (which stay hotter longer than more rural areas).

Thanks! We are going July 10-24, 2019

The list of places you'd been before definitely includes some warmer spots. There are countless options that'd be a lot cooler, even in summer. As has been mentioned, there's always the possibility of an unusually warm day or stretch of weather, but ultimately all we can do is play the odds. London can be a good starting point for lots of other places around it, and you can even take a pleasant train ride up to Edinburgh and spend time up there. Prague is also pleasant in summer - averages won't normally reach in to the 80s F there in summer.

So I'd mainly just avoid the Med - much of Spain, Greece, and lots of Italy ruled out of wanting to avoid warm weather; southern France, in and around the Adriatic may not be to your kids' liking (but even then, we're playing the odds as the right spot in the right coastal town can be quite pleasant even in summer).

If you're all fans of London, perhaps a quite re-visit, then branch out to other parts of England and Scotland and Wales from there? No shortage of charming villages to spend time in throughout.

Indeed - you are planning to go during the absolute peak of the peak season (in terms of busy-ness, not necessarily weather).
It's getting late for the more popular places. Time to make some choices and start nailing things down (like flights!).

Nordic or Baltic countries.

My interest has recently been piqued by the Polish Baltic coast, it's certainly on my to do list but trying to fit it in somewhere is the difficulty at the moment.

https://www.theguardian.com/travel/2016/sep/14/sand-sopot-poland-riviera-hel-peninsula-gdansk

SWITZERLAND!! The Alps are heavenly perfection in July! We so enjoyed Murren last year mid-July. (And we actually needed a coat at night.)

Yikes, you are really late in your planning. I planned a month long trip to Europe last summer with our two girls.
We did Norway, Scotland and Ireland.

Oslo was having a heat wave! I concur with others that staying north is your best bet for cooler temps.

Yes, your recommendations are great!! Andi will get on booking this trip this week. Thank you!