I flew at Xmas when my daughter was exactly that age, and the trip over she did great, the trip back she cried most of the time. But we had a much longer flight. You should be fine. What seemed to calm my little one down was giving her a bottle with just water. I know you normally shouldn't do that, but it's fine for a couple of hours. Mine loves water for some reason. And people told me to take the Boppy too, but i decided that was just too much to carry on, so when she napped in my lap, I just asked the stewardess for extra blankets and pillows for cushioning.
As far as the time change, I just split it in half. our time chg was 4 hours, so I made it 2 hours. But dallas isn't that far, so dont' worry about that at all.
But looking back, I definitely worried about it more than I should have. Also, I never drink on flights, but I did on that one! That can help too if you are stressed. You can offer to buy the people around you a drink if you are feeling generous.