Earring is signed by holding and releasing your ear between your index and middle fingers and your thumb (or sometimes just your index finger and thumb). The sign looks like you are touching and releasing an imaginary earring on your ear.
A third, alternative ASL sign for earring is using both hands touching both ears at the same time.
Babies are often fascinated by earrings worn by the adults in their life, wanting to touch at the earring. You can use this natural interest to teach them the sign for earring. Knowing the sign also lets them express their interest without grabbing.
Baby girls, particularly those with pierced ears, are very interested in earrings. You can use the sign to describe the accessory and also to teach your baby not to pull your earrings.