I always like to get there in time to eat breakfast, plus the security line takes longer taking everyone's shoes off and all that. You start boarding at 6:30 am. I personally would get there at 5:15/5:30 to have time for breakfast. My child has a food allergy, so we can't eat just whatever. And I like them to get some good food/protein in them before the flight.

1418++ You CAN get delayed in line sometimes. I would arrive 530-545 just in case. Worrying about a line or delay stresses the hell out of me. So what if I get there 'early' enough to find an actual seat by the gate, read a newspaper or browse DCUM if I have plenty of time

If you check in online, 6am is fine (unless you're parking). Shouldn't be very busy. We fly with a 2.5 year old pretty frequently out of DCA , get there one hour before and usually have about 10 extra minutes at the gate before boarding.