Muscular dystrophy: This condition, particularly Duchenne muscular dystrophy, often presents with proximal muscle weakness, which could explain the difficulty in standing up from a sitting position (Gower's sign) and sitting up from a lying down position, despite being able to walk without support. The age and gender of the child, along with the specific pattern of weakness, make this a strong consideration.

Congenital myopathies: These are a group of muscle disorders (e.g., nemaline myopathy, central core disease) that can present with muscle weakness, which might fit the child's symptoms, especially if there's a history of delayed motor milestones.
Spinal muscular atrophy (SMA) type III: Although SMA type III (Kugelberg-Welander disease) typically presents later in childhood, it can cause proximal muscle weakness, which might result in the described symptoms, especially if the disease onset was relatively late.

Spinal cord injury or compression: Conditions such as spinal cord tumors, abscesses, or traumatic injuries could lead to muscle weakness. Although the neurological examination is reported as normal, subtle signs might be missed, and the consequences of missing such a diagnosis could be severe.
Inflammatory myopathies: Conditions like juvenile dermatomyositis could present with muscle weakness, and it's crucial not to miss these diagnoses due to their potential for significant morbidity if left untreated.

Metabolic myopathies: Disorders such as Pompe disease or debranching enzyme deficiency (Cori's disease) can cause muscle weakness. These are rare but important to consider, especially if other diagnoses are ruled out and the child's condition progresses or doesn't improve with standard treatments.
Mitochondrial myopathies: These are a group of disorders affecting the mitochondria, which can lead to muscle weakness among other symptoms. They are rare and might require specific diagnostic tests to identify.