Mucocoele: This is the most common cause of a bump under the lip in children. It occurs when a minor salivary gland becomes obstructed, leading to the accumulation of mucin and the formation of a painless, translucent, or blue-colored swelling.

Traumatic hematoma or bruise: Given the age of the child, accidental trauma to the lip or mouth area is common and could result in a bump or swelling under the lip.
Dental abscess or infection: An infection related to a tooth, especially if it's a primary tooth that's erupting or has erupted, could cause swelling in the lip area.
Soft tissue cysts (e.g., dermoid cyst, epidermoid cyst): Though less common, these could present as a bump under the lip.

Rhabdomyosarcoma or other soft tissue tumors: Although extremely rare, these malignant tumors could present as a swelling or bump under the lip. Early detection is crucial for effective treatment.
Infectious diseases (e.g., tuberculosis, actinomycosis): Certain infections, though uncommon, could cause swellings in the oral cavity, including under the lip, and have significant health implications if not treated.

Lipoma or other benign tumors: These are rare in young children but could present as a bump under the lip.
Congenital anomalies (e.g., congenital granular cell tumor): Some rare congenital conditions could cause a bump or swelling under the lip in a child.
Foreign body reaction: If a child has inserted an object under the lip, it could cause a reaction leading to swelling, though this would typically be accompanied by other symptoms like pain or difficulty speaking.