Cucumbers

I am new to gardening and am growing for the first time. I have some cucumbers grown indoors from seed. they are now about 6 inches high and ready to plant out. Is it too early (too cold) to do this now? Also, the seed leaves have started to dry out although the plant leaves appear to be fine. Is this normal or have I over watered them?

We tried growing Q's  from seed this year but now they are 2-3 inches high some have started wilting. Have they been over watered or is it too cool? They are on a north facing windowsill. Wish I'd saved the money it cost to buy 2 packets of seeds (indoor and outdoor ones) and bought a nursery plant later on.

Yes they are on a north facing window sill. The rest of the plant seems to be thriving and it is only the seed leaves that appear to be affected at the moment. I will reduce the watering and see if it makes a difference.