Depending on the variety they should be able to handle those temps. I am up here in Ontario Canada and last night my Redwing and Cortland seedlings were exposed to -6.5C (20F) in my unheated greenhouse. They are in cell packs and the grow medium just about froze solid and they survived and appear to be fine now. I don't know if this will adversely affect them later in their growth ie. bolting or thick necks, I guess I'll find out. We are having another very cold night tonight so I brought them inside, I don't want to push my luck.
They are about 9 weeks out from seed in 100 cell packs, about 2 - 4 seedlings per cell.
Hope this helps.

Freezing temps should not be a problem. But I would try to mulch them with dry leaves/straw/ pine straw heavily to prevent from repeated short cycle freezing and thawings, as much as possible. This is the only reason for mulching the over-wintering garlics as well. The ground (in zone 5) will probably freez to a depth of more than a feet with or without mulch.