According to American Youth Soccer Rules, metal and plastic hair clips, barrettes, bobby pins or hair bead barrettes, are not allowed on the field for practice or during a game. Players with long hair may wish to wear soft hair ties such as 'Scrunchies, hair ribbon, soft bands and elastic hair band.
A player must not use equipment or wear anything that is dangerous to themselves or to other players. All items of jewelry (necklaces, rings, bracelets, earrings, leather bands, etc.) are forbidden and must be removed.
If it can come off, it should come off.
No Body jewelry includes earrings, rings, belly rings, bracelets, watches, and necklaces; it also includes hair pieces, such as barrettes, bobby pins, hair beads.
Essentially, anything hard and with the potential of harming the player or others on the field is not allowed due to safety precautions.