I would not count on UA holding the plane for OP - even if the flight attendant or gate agent on the arriving flight say they will try. Unless instructed by someone higher up, the departing flight's agent's focus is on-time departure.
It is definitely a good idea for OP to have the United App installed and log into the account that is flying. Connect to the Wifi on the plane and use the United App for free. If the ATL-IAD flight is late and OP will misconnect, the App might pop up a yellow/red bar and help OP rebook. Still with 4 people and not much choice... OP can also track bags on the United App.
If OP gets moved to other carriers, also make sure the agent has re-validated the ticket and OP should get them to print out the new itinerary and ticket receipt.
Also, might be a good idea to make sure the return flights are intact a few days after arriving PEK if OP is rerouted on another carrier.