Third Cranial Nerve (Oculomotor Nerve) Palsy: This condition is the most likely diagnosis due to the presentation of ptosis (drooping eyelid) and internal strabismus (inward turning of the eye), which are classic signs of oculomotor nerve palsy. The oculomotor nerve controls several muscles that move the eye and also controls the levator palpebrae superioris muscle, which lifts the eyelid.

Congenital or Acquired Brown Syndrome: Although less common, Brown syndrome can cause strabismus, but it typically presents with limited upward gaze and is not usually associated with acute onset ptosis.
Monocular Elevation Deficiency: This condition could potentially cause strabismus but is less likely to present acutely with ptosis.

Aneurysm of the Posterior Communicating Artery: Although rare in children, an aneurysm compressing the third cranial nerve can cause sudden onset of ptosis and strabismus. Missing this diagnosis could be catastrophic.
Diabetic Third Nerve Palsy: Though more common in adults, children with undiagnosed diabetes can present with third nerve palsy, which would require immediate medical attention.
Intracranial Mass or Tumor: Any mass effect in the brain that compresses the third cranial nerve can cause these symptoms. Early detection is crucial for treatment and prognosis.

Graves' Disease: While primarily known for causing exophthalmos, in rare cases, Graves' disease can cause restrictive strabismus and could potentially mimic the presentation, though ptosis would be less common.
Myasthenia Gravis: This autoimmune disease can cause fluctuating muscle weakness, including eye muscles, leading to strabismus and ptosis, but it's rare in children and the presentation is usually more variable.
Botulism: Botulinum toxin can cause bilateral ptosis and strabismus due to its effect on neuromuscular junctions, but it's exceedingly rare and usually presents with other systemic symptoms.