Take a small butter dish lid and spread Elmer's glue in it pretty heavily.
Give the kids the floss and have them lay it in the glue in pretty patterns. Long pieces, short pieces. Whatever looks good to them.
Leave one good sided loop of floss up and over the edge of your lid.
When they've filled it in mostly with floss, cover it with more glue.
After the glue dries, pop it out of your lid and you have a Christmas ornament.
As long as they don't get wet, they will last for years.