I was going to ask where you're going, but looked at your home page -
"2024 meeting friends in the northeast France / Belgium area July 5 - July 12. Flying Denver to Reykjavik June 8th (arrive 9th) and back to Denver from Reykjavic July 13th. Currently have tix from Paris to Reykjavic July 12th. Insistent on seeing Iceland, Barcelona and Paris."
I never had a problem getting breakfast at 7:30 (croissant & espresso) in Paris when I worked there decades ago, and I imagine it's the same elsewhere, locals have to get to work so some places will be open. You can look up things on Google maps, in fact I would start a 'saved' map for each of your destinations as you find places to eat / munch along with things you'd like to see when you're out and about. As a general comment, the further south you venture the later in the evening people eat dinner, HOWEVER, you can usually find places that open earlier, especially in the tourist centers where some people like to eat earlier. I would check the blogs for Barcelona & Paris. We had no trouble getting fed at 5PM, or a bit later in Rome over NY, so I suspect the same would be true for your destinations but ask at your hotel. I was shocked to be fed so early in Rome. And there's always the appetitive bar for a lighter dinner, seem to recall that's what we did in Spain.
Personally, I would go to places with the maximum crowds as early as you can drag the teens out. For example the Louvre museum, Orangerie & Eiffle Tower, prior tickets are a big plus. If there are areas you just want to stroll, go ahead and plan on heading out early, you may have some of them to yourselves. No comment on Barcelona, looks like the Segrada Familia opens at 9AM according to the website but wonder if there are masses you could attend earlier?? Some churches open early and have great services, soak up some ambience! A very brief search pulled up these ideas. No comment on Iceland.
https://community.ricksteves.com/travel-forum/france/early-morning-coffee-and-croissant-in-paris
https://community.ricksteves.com/travel-forum/france/early-morning-coffee-and-croissant-in-paris