Hygiene or Environmental Factors: Poor hygiene, infrequent bathing, or wearing the same clothes for extended periods can lead to excess armpit odor in children. This is the most common and likely cause, given the age of the child and the simplicity of the solution (improving hygiene practices).

Hyperhidrosis: A condition characterized by excessive sweating, which can contribute to body odor. In children, it can be primary (without an underlying cause) or secondary to another condition.
Dietary Factors: Consuming foods that are high in sulfur (like garlic or onions) or spicy foods can make sweat smell worse.
Hormonal Changes: Early onset puberty can lead to increased apocrine gland activity, resulting in stronger body odor.

Diabetes: Although less common in children, uncontrolled diabetes can lead to increased glucose in sweat, which bacteria can feed on, producing a strong odor.
Thyroid Disorders: Hyperthyroidism can cause excessive sweating and potentially lead to stronger body odor.
Infections: Certain infections, such as those affecting the skin or lymph nodes in the armpit area, can cause a foul odor.

Trimethylaminuria (Fish Odor Syndrome): A rare genetic disorder that prevents the body from breaking down trimethylamine, a compound found in certain foods. This leads to a strong fish-like odor in sweat, urine, and breath.
Isovaleric Acidemia: A rare metabolic disorder that can cause a sweaty foot odor due to the accumulation of isovaleric acid in the body.
Other Metabolic Disorders: Certain metabolic disorders can lead to specific odors due to the accumulation of particular substances in the body, though these are exceedingly rare and usually present with other significant symptoms.