I'd like to assemble our gingerbread house in the evening, to allow the structure to harden before my kids decorate it. Will the royal icing (2 egg whites/5 C icing sugar) keep overnight in an airtight container, such as a ziploc bag?
Yes, as long as no air can get to your icing it should be fine---though you may need to stir it up---I have heard that royal icing made with egg whites doesn't hold up as well as royal icing made with meringue powder, but give it a try.
