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?)
> Only multi-engine, fixed-wing aircraft up to Airbus A318 size with special aircraft and aircrew certification to fly 5.5° approaches are allowed to conduct operations at London City Airport.
Looks like smaller jets (E190's mostly) due to the very steep approach on both ends, but the views look pretty good and there's decent traffic there.