Depending on where you are and what you want to donate -- Head Start programs can always use hats, mittens, small afghans (all day programs), and even infant things if they have an Early Head Start program. The children are low income, so they really can use that type of stuff. It isn't just preschool, but daycare in some programs as well (why they need the afghans for naptime).
When I lived in Maine & worked for EHS, there were some ladies in the area who would knit for the children -- at Christmas time they would get outfitted for winter and when they graduated we let them keep their favorite afghan. Some all day programs help with clothing, too.