I think I'd be wary of moving them outside once they'd germinated, if they germinate outside it means it's warm enough, otherwise they wouldn't germinate! Waiting until they're up and then putting them outside in the growhouse might be a bit harsh. It just depends what the weather does.
I think if I were you, I'd move them outside ASAP, they will then germinate when it's warm enough, it might take longer, but I wouldn't risk losing everything later on if there's more snow on the way. Waiting is difficult, it seems to have been winter a long time this year.