When we visited in April, our youngest (5) had perpetual residue on her fingers from whatever the last snack she ate was, so her finger scans were never consistent. Eventually, they just had a cast member take a picture of her with an iPad and then that was used to verify identify for entry. So that is always an option as well. As others have stated, you can also use a parent's fingerprint, too; but then that parent always needs to be present with them for them to enter the park. But if they aren't there, it's no big deal, they will just take a new scan or do a picture.