The bus is going to take a very long time. If time is of the essence you should consider using a taxi.
Going from Epcot to DTD, you can walk out the International Gateway and catch a bus from either the Beach Club or Boardwalk (BC is closer but the bus will head to DTD from BW so it's not really a problem which one you pick).
Coming back, that bus will stop at Swan, Dolphin, Yacht Club, Beach Club, and Boardwalk. So the resorts you need to reach for an easy walk back to Epcot are the last two on the list (if you go that route use the Beach Club, it's a bit closer to Epcot). You should allow at least an hour and a half to do this.
Plus, if Typhoon Lagoon is open, throw in an extra stop there on both legs of the trip.
Call T-Rex directly and get a better time rather than deal with the bus when trying to go from Epcot to DTD and back. It is going to be a LOT of time on a bus with 3 little children.