If you have cardstock (heavier paper) you can print on you can print up tags that say something like -
"Hand wash ONLY. Washing in the washer and dryer will shrink and felt this hat"
Then you can punch a hole and use a holiday sticker or stamp to make it cute and Christmassy. OR if you're handy with the computer and have software you could create a tag with a border and text and print it...
ETA: I just made these labels up in about 5 minutes. If you have the software and know how to use it you could make up your own.