Toddler's fracture: A common injury in toddlers, often resulting from a minor trauma or stress fracture, which can cause significant pain and refusal to bear weight. The age and presentation make this a highly plausible diagnosis.

Infectious causes (e.g., osteomyelitis, septic arthritis): Bacterial infections can cause severe leg pain, fever, and an inability to bear weight. These conditions are more common in young children and can be serious if not promptly treated.
Trauma (e.g., fractures other than toddler's fracture): Other types of fractures or soft tissue injuries from falls or accidents can also lead to leg pain and an inability to bear weight.
Transient synovitis: Although more commonly associated with hip pain, transient synovitis can sometimes cause leg pain and is a consideration in a child who is refusing to bear weight.

Malignancy (e.g., neuroblastoma, osteosarcoma): Although rare, malignancies can present with leg pain and must be considered to avoid delayed diagnosis and treatment.
Non-accidental trauma: It's crucial to consider the possibility of child abuse, especially if the history provided does not match the injury or if there are other suspicious findings.
Slipped capital femoral epiphysis (SCFE): More common in older children but can occur in younger ones, especially if there's an underlying condition affecting bone health.

Legg-Calve-Perthes disease: A rare condition characterized by avascular necrosis of the femoral head, which can cause hip and leg pain.
Juvenile idiopathic arthritis: An autoimmune condition that can cause joint pain and swelling, including in the legs, though it's less common to present with an acute inability to bear weight.
Neurological conditions (e.g., peripheral nerve injuries, spinal cord lesions): These can cause leg pain and weakness but are less common in this age group and presentation.