I work in an NICU and we can use caps of all sizes from 10 inches on up. They do have to be washable,soft, no bumpy trimming or decorations, and please wash without fabric softener as the smell can be irritating to little noses. For the smallest sizes it is best to use finer yarns and smaller needles. We get knitted and crocheted caps, and for all we know, some of them may have been made on a knitting loom.
You might take them to the hospital to have them checked out.
All the other charity ideas are great, too. We can do so much good with our knitting.