L.D.
For some children, their mouth may feel undersensatized (much like having novacaine in their mouth constantly), so they seek out sensation by putting non-food items in their mouth a lot. The term for this is called sensory processing disorder (SPD) and a occupational therapist who is experienced with treating SPD can help your daughter regulate her senses so that this will become a non-issue. Something that you can do right now is giving her a battery operated teether or electric tooth brush to put into her mouth each time you see her mouthing a non-food item.
Wishing you and your daughter all the best. If you have any other questions, please feel free to e-mail me.