Maybe there is some way, you can try to use a larger sized bed sheet, tented over them, so they don't get too cold, at least until after it warms up a bit, the next day ,. You don't want the weight of the sheet to bend, or break the tiny stems & may need some help, to do it properly, without harming them. That is rather a bit chilly for Zinnia seedlings, if they've never experienced temperatures nearly quite that cold,, before...
