Often baby bracelets and other children's jewellery are worn only for special occasions. This can mean the jewellery is stored in a jewellery box or cupboard much of the time. If this happens for lengthy periods, the sterling silver can tarnish.
Some manufacturers coat their sterling silver jewellery in a hardener, such as platinum, or Evershine which comes under various brandnames. Basically, this prevents scratches, the main cause of dulling of the silver. It also prevents tarnishing. So, if you come across kids' jewellery that has either of these coatings, it's even better value.
Wearing the jewellery is the best way to prevent tarnishing. But if the jewellery is to be stored, place it first, into a small zip lock plastic bag then into the cupboard, jewellery box, etc.
If you find tarnishing is a problem over long periods, simply purchase a jeweller's cleaning cloth and give your little baby bracelets, kids bangles etc. a good clean. Alternatively, the jeweller will do this for you for a small fee, and often for free, as a goodwill gesture.