Gamekeeper's thumb (Skier's thumb): This is a common injury in children who participate in sports, especially those that involve falling onto an outstretched hand, such as trampolining or dancing. The presence of a small bump and swelling, particularly after a recent increase, suggests a possible ligament sprain or tear, which is consistent with Gamekeeper's thumb.

Thumb sprain: Given the patient's history of sports and dance activities, a sprain from overuse or a specific injury is plausible.
Ganglion cyst: Although less common in children, a ganglion cyst could present as a bump and cause pain, especially if it becomes inflamed or irritated.
Osteochondritis dissecans: This condition involves a piece of cartilage and bone becoming detached, which could cause pain and swelling, particularly if the patient has experienced repetitive stress or trauma to the thumb.

Infection (e.g., osteomyelitis or septic arthritis): Although less likely, an infection could present with pain, swelling, and redness. Missing this diagnosis could lead to serious complications, including bone or joint damage.
Bone tumor: While rare, a bone tumor could cause persistent pain and a palpable mass. Early detection is crucial for effective treatment.

Ehlers-Danlos syndrome: This genetic disorder affects connective tissue and could lead to joint hypermobility and increased susceptibility to injuries, including those affecting the thumb.
Sickle cell disease-related dactylitis: If the patient has sickle cell disease, dactylitis (hand-foot syndrome) could cause thumb pain and swelling, although this is more common in younger children.