Mother's Day Brunch w/ Kids?
I see kids at Dupont Grill in Dupont Circle sometimes. Perhaps they would like Firefly (although its a smaller restaurant so if they tend to move around a lot, that might not be a good choice), Bistro Francais in Georgetown.
Opentable has a big list of Mother's Day brunches you might want to check out. I recommend making a reservation ASAP though.