Love this thread and seeing the amazing trips everyone's doing with kids - we travelled with just one child, but looking for something easier for the next few years now we have two (and littlest one is a bit more of an erratic travel companion than the first!)
Singapore/Gili Islands - Singapore is amazing with kids, so much to do (not just a stopover) and Gili Islands are amazing. This part of SE Asia is good weather in Aug too (not rainy season). Many people go from Bali, but closer from Lombok, really beautiful.
Vancouver - very cool city to stay a few days, then we rented a car to go to Vancouver Island. Lots of nature opportunities, whale watching, etc. We stayed in a town called Tofino, really tiny but PNW is just beautiful, people are super chilled. From Vancouver you could also spend a few days in Seattle or Portland - big food scene, would be lots of fun with teens.
East Coast Canada - we have flown into Toronto to visit family, and Niagara is a great day out/night. From there you could go on upstate New York or drive east in Canada and visit Quebec and onto New Brunswick. It's a lot of driving though!
Lived in USA as a child and my dad planned amazing trips in California/Wyoming in a RV camper van - you can do a few days at Disney and then spend the rest in National Parks - Sequoia, Joshua Tree, Yellowstone etc.