I grow lots of seedlings in mini greenhouse, and the point is to make them grow fast in the warm and moisty environement, and when the seedlings are big enough, then i start to introduse them to a "normal" environement. I have the greenhouse indoor, and moist the seedlings at least twise a day, more if needed!
First i just open the greenhouse a few ours in the warmest part of the day, to let the temprature and the humidity drop a bit, the seedlings are still in the greenhouse, with a shelter from the strong spring-sun. After doing this a few days, i starts to keep the house open all the time during the day, and then after a few days also at night. Still the seedlings are in the house.
When the seedlings have got leafs, not only the first "baby-leaf", but tiny real leafs, then i put them directly outside, keeping them in a sheltered place not to get burned by the warmest sun in the middle of the day, and not where there is direct wind. Its important that there no danger of freezing at night, dont take them out before its warm enougt!
Remember to mist them only in the morning before the sun reaches them, and after the sun has passed, or they will get burned and die! They can stand a little cold, but not to warm, or dry out...
This is the way i grow most of my seedlings, from summer-flowers to little halloween pumpkins, and they all seems fine, growing willingly outside as we speak.
I still need to learn how to grow orchids, thoug!