If you're only going for 4-5 days then I'd skip the West Coast, it's an extra 4 or 5 hours in the air.
Toronto is great, but obviously I'm biased. Depending on what you like to do, the downtown tourist plaza has a ton of things to do with the kids - the CN Tower obviously, but also there's a brilliant aquarium, a big entertainment centre called the Rec Room which has restaurants and bars and a huge arcade filled with games, and when Dad needs some time alone the Steam Whistle Brewery does a really fun guided tour complete with beers along the way. It's all right on the waterfront which is super scenic and you can get a 10 minute ferry or water taxi over to the islands which are beautiful and give you a stunning view of the skyline. There's also a ton of cool neighborhoods and markets dotted around, and our transit system is pretty good.
I obviously don't know Boston or Chicago so well, I'll let our resident experts give you the skinny on those but I also love both of those cities. I don't think you'll go wrong on any of the three tbh.
I also love DC but I don't know if it's worth a family holiday, and October is just before the midterms so I would have a mild concern over protests, rallies, and an excess of jackbooted security. Probably recommend skipping that to be on the safe side.