I recently found out you can do that on one of the roads to the entrance of the Smithsonian Udvar-Hazy museum in Dulles. Nowhere near as Gravely Point, but it will do in a pinch.
For Logan airport in Boston, Revere Beach is one place with a pretty good view of airliners, and is short walk from the Blue Line.
You can also see a bunch of other places on a map that line up with runways: South Boston (including Castle Island), East Boston, and a bit further from Logan is the harborwalk around Long Wharf and the aquarium. (The aquarium might also be interesting to kids. There's some free viewing windows for a little of it, while walking past, to maybe gauge interest before paying admission. And there's various tourist things in that area, including various harbor boat rides.)
A camera/cameraphone with >1x lens will get them a closer look at planes. Or binoculars/monocular if you want to reduce screen time, or to encourage appreciating it in the moment rather than trying to capture it. (What kind of hearing protection works for kids?)
Going to Pearson airport in Toronto with my dad is one of my few early childhood memories. I don’t know exactly where we were but it was a popular enough spot that someone had set up a hotdog stand. In my mind it very clearly registers as a scene from the 80s. The aircraft everyone was hoping for was the 747. I still remember the roar of that thing coming in.
Rush fans will know this, but part of the HN crowd won’t and likely find it interesting: there’s a musical connection between Rush’s YYZ song and the Morse code identifier transmitted from a nav beacon at the Pearson Airport (IATA code YYZ)
