Repetitive Strain Injury (RSI) or Tendinitis: Given the history of repetitive use and the presence of numbness in the hands and lower inner forearm, RSI or tendinitis is a plausible diagnosis. The symptoms are consistent with overuse of the wrist and forearm, leading to inflammation of the tendons.

Ulnar Nerve Entrapment or Neuropathy: The numbness in the lower inner forearm and hand could be indicative of ulnar nerve compression or damage, which can occur due to repetitive strain or direct pressure on the nerve.
Ganglion Cyst: A ganglion cyst, which is a lump under the skin that occurs near joints or tendons, could cause numbness and pain if it compresses a nerve.
Wrist or Forearm Strain: A strain in the wrist or forearm muscles could also lead to numbness and pain, especially if the strain affects the nerves running through these areas.

Thoracic Outlet Syndrome (TOS): Although less common, TOS could cause numbness in the hands and forearm due to compression of the nerves and/or blood vessels that pass through the thoracic outlet. Missing this diagnosis could lead to prolonged and unnecessary suffering.
Cervical Radiculopathy: Numbness in the hands and forearm could also be a symptom of a cervical spine issue, such as a herniated disk or bone spur, compressing a nerve root. This condition requires prompt attention to prevent long-term damage.

Vitamin Deficiency (e.g., Vitamin B12 Deficiency): Certain vitamin deficiencies can cause numbness and neurological symptoms. Although rare, it's an important consideration, especially if the patient's diet is deficient or if there are absorption issues.
Multiple Sclerosis or Other Demyelinating Diseases: These conditions can cause a wide range of neurological symptoms, including numbness. They are rare, especially in a 12-year-old, but should be considered if other explanations are ruled out and symptoms persist or worsen.