And for smaller children, in cities like Amsterdam, it’s common to see a mom riding with multiple kids sitting in a cargo bike.
Note that the maximum grade for a wheelchair ramp is 8.3%.
By the way, i visited Chicago, Michigan and Colombus, Ohio: I might now have seen everywhere, but those are closer in elevation difference to Amsterdam than Paris and Lisbon are
From what I recall of Columbus, it is every bit as flat as you say. I'd have a hard time telling you about a hill in Chicago. Then again, I'd have a hard time telling you about a time that I was on something other than an 6+ lane highway in Chicago. The view you get from a major highway is often different from what you get in areas where you think of your kids having a casual bike ride.