Romaine lettuce, chopped finely should be okay. My ducks have trouble with lettuce, though. I would recommend using peas, you can give peas to them at about 1 week, and they LOVE them!
If you avoid harder foods like squash or carrots and are willing to chop the food to small chunks then you can give pretty much any duck-safe foods once they're a week old. I chopped up blueberries for Moxy and Norie when they were a week old. (you need a really sharp knife to chop berries and not end up with mush. Lol )
Just remember to give treats in moderation so they'll continue to eat there main feed instead of holding out for the good stuff!