We do a lot of walking in cities, sometimes for miles, and I have a strong preference for traffic-free pedestrian zones.
Copenhagen, with its long Strøget ( pedestrian street) would top my list for a family destination. It is a lovely and very friendly city, with nice parks and the Tivoli amusement park. Lots of English spoken.
Munich has a nice long pedestrian zone as well, and the Englischer Garten is nice to stroll through (;and watch the surfers taking on the standing wave on the river).. The Deutsches Museum is great for teens.
In London, we love connecting three parks—Hyde Park/ Kensington Gardens, Green Park, and St. James Park—- to make our way from Paddington or Knightsbridge to the area around Westminster Abbey. There are couple of major streets to cross, but the rest is all on park paths.
Also in London is the Thames Path along the South Bank. We have used this wide pedestrian path to cover the distance from Chelsea upriver all the way down to Greenwich (not in one walk). The part between Westminster Bridge and Tower Bridge is particularly nice, lined with shops and restaurants, and great views across the river. Cross on the pedestrian Millennium Bridge to reach St. Paul’s from the Tate Modern.
When you tire of walking, London buses are stroller-friendly and much less expensive than the Tube ( and the tube stations often involve steps, though one can get a map showing tube stations with step-free access).
And then there is Venice, all of which is a “pedestrian zone”. But the bridges that cross all the little side canals can indeed present a problem for strollers. Two parents ( or a parent and a teen) could pick up the stroller to lift it up and down the little bridges, but for the larger ones like Rialto and Accademia you would probably want to take the child out and carry her separately from the stroller. Or bring a child carrier backpack instead of a stroller.
We were last in Venice in September, just for an afternoon before our flight to London the next day. We walked the length of the Zattere, the wide sidewalk bordering the Giudecca canal, all the way to the end at Punto della Dogana ( our favorite viewpoint in Venice). All the bridges along there had ramps added to make them more accessible to strollers and wheelchairs. So this part at least you could manage with a stroller. Maybe they have added the ramps elsewhere as well.