Idiopathic thrombocytopenic purpura (ITP): This condition is characterized by low platelet count, which can lead to bruising and bleeding. However, given the specific symptoms of leg weakness, foot edema, and discoloration, it's less likely but still a consideration due to the potential for bleeding into muscles or joints causing weakness and edema.

Cellulitis: An infection of the skin and subcutaneous tissues, which can cause redness, swelling, warmth, and pain in the affected area. The symptoms of leg weakness could be secondary to pain or discomfort.
Deep Vein Thrombosis (DVT): Although less common in children, DVT can cause leg swelling, pain, and discoloration. The weakness could be due to pain or decreased mobility.
Musculoskeletal injuries: Such as strains or sprains, could lead to localized swelling, pain, and weakness, especially if the injury involves significant soft tissue damage.

Compartment Syndrome: A serious condition resulting from increased pressure within a muscle compartment, which can lead to muscle and nerve damage. Symptoms include severe pain, swelling, and weakness in the affected limb.
Neurological conditions (e.g., Guillain-Barré Syndrome): An autoimmune disorder that can cause muscle weakness, which often starts in the legs and can ascend. Although less common, it's critical to diagnose early due to the potential for rapid progression and respiratory compromise.
Osteomyelitis or septic arthritis: Infections of the bone or joint, respectively, which can cause pain, swelling, and decreased mobility of the affected limb, potentially leading to weakness.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. While it can cause easy bruising and joint hypermobility, it's less directly linked to the acute presentation of leg weakness, foot edema, and discoloration.
Kawasaki Disease: An illness that affects children and causes inflammation in the walls of medium-sized arteries throughout the body. It can lead to symptoms like redness and swelling in the legs, but the classic presentation includes fever, rash, swelling of the hands and feet, irritation and redness of the eyes, lips, throat, and tongue.
Vasculitis (e.g., Henoch-Schönlein Purpura): Inflammation of the blood vessels, which can cause a variety of symptoms including purpura (bleeding under the skin), joint pain, and gastrointestinal symptoms. While it can present with leg weakness and edema, it's less common and typically accompanied by other systemic symptoms.