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?)