I'm often asked about sizing on baby bracelets especially, but also children's bracelets. It's difficult because babies vary in size enormously at the same age so it's always best to measure her wrist. Same goes for kids' bracelets. Teens are usually small women's sizes though even women's sizes vary a lot.
So, without benefit of an exact measurement, which is often the case in the event of purchasing a baby gift or a child's birthday gift, here is a guide. If your recipient is large for her size, go one size up. If she's small for her age, go one size down. For babies and little girls especially, look for bracelets with an extension for growth and bangles which expand. This will extend the time the bracelet or bangle can be worn.
For keepsake style baby jewellery, consider buying her, her first jewellery box to keep her gifts safe and secure, where she can store them until she grows up and has her own children.