I would need to know more about your coldframe. Does it have a automatic vent system to open when the sun raises the temperature from 50 to 90? The 50 degrees night time temperatures would not be a problem for your lettuce, but I think I would wait until the seedlings were at least an inch or so high. Al

What are your temps outside right now? It may be better off just getting them in the ground instead of in a cold frame. I would worry about you cooking them in the coldframe like Al had said.

Lettuce love 40-60* I think it will be too hot during the day unless you prop open your cold frame or just set them in the ground.

I would harden them off ASAP and get them outside. Al's way would work too, after 1" of growth, I just think lettuce tastes so much better when outside in the chilly air!

Keriann~