Dystonia is a common movement disorder in children. It has many causes such as those related to birth injuries (hypoxic - ischaemic brain injury, severe jaundice, stroke, meningitis etc), metabolic disorders, hereditary or genetic disorders, toxins etc.
Further, the dystonia can affect one part of the body or can affect the whole body. Hence, the treatment is going to depend upon 1) the cause, 2) the location - single part, many parts or whole body, 3) the severity of the disease.
Regarding MRI imaging, you have not mentioned what implant. Pacemakers, intracranial metallic implants are contraindications for MRI. A dental brace may cause some image artifacts to occur; but if the brace can be removed, then getting an MRI head is more informative.