What kind of seedlings are we talking about? Yes I think you can take them out in the daytime. I would start them out in the shade since the outside daylight is much stronger. Also if it gets windy make sure to shelter them since they are still delicate babies. After several days of shade gradually start giving them more sun until they are in full sun. If it gets into the 30's definitely bring them in unless its something like peas or lettuce etc.

Beware of putting them into a small greenhouse in the daytime it can get really hot in there when the sun is on them. You can probably manage without it as long as you can protect them at night.