Do you need a hotel suggestion or suggestions for activities? Udvar-Hazy air and space museum would be ideal but unfortunately closes for the day at 5.30. By the time you deplane, check in to your hotel, etc, will probably be too late.

There are a bunch of hotels around the airport that provide shuttles. I think your best bet is to get figure out how to get help. How much luggage and how many grown ups? Can you have your luggage booked through, or do you need to pick it up at Dulles? Plan to ask the hotel staff for help and to curb check luggage, and make sure you bring a bunch of dollar bills to tip. Can you put the 3 yo in an umbrella stroller and the 1 yo in an front carrier, and then carry your carry-on gear in a backpack. Maybe with a front cross-body so you can reach ID, cash, etc?

Are you changing time zones as you fly (either before IAD or after?)