Something families keep, not just buy
Each child draws a simple Christmas design, which gets printed onto cards and sold in packs to parents and family. Unlike most fundraising items, this is something families genuinely want, because it's their own child's artwork.
Printing costs are the only real expense, and there are printing services that specialise in exactly this kind of school fundraiser, often at a reasonable price per pack. Selling packs of ten or twelve cards works well as a Christmas gift for grandparents.
It doubles as a nice keepsake alongside being a fundraiser, which tends to mean better uptake than a purely functional sale.