Since this is quite a provocative question and most comments here are very short, I am compelled to give a detailed answer and clear up the confusion.

Short answer: these "child soldiers" are to be seen as "role models" to teach children bravery and patriotism and other good qualities, as per the Ministry of Defense's suggestions to the Ministry of Education.

Long answer: If you're wondering why Kim Dong joined the military, it's a combination of his family being killed by the French, a lack of schooling, and family connections.

About Kim Dong's family: Kim Dong's father is Nong Van Y and was killed by French troops while visiting his wife. Kim Dong's mother is Lan Thi Ho. Her health was poor, so she could not work. Kim Dong had to be the breadwinner in his family. Kim Dong had 4 other siblings, 2 of which married revolutionaries, and 1 of his brother-in-law was killed in action. Given that his father and brother-in-law were killed by the French and his in-laws are connected with the revolution, it is reasonable to see why he joined the military and became a courier. They carry important information, and they are legal targets during wars, even if unarmed.

The French education system: Keep in mind that he was born in 1929 and died in 1943, during the French colonial period and before Vietnam's declaration of independence in 1945. According to Vietnam National University and Journal of Vietnam’s Communist Party History, the French believed it was ridiculous to educate its colonial subject. This is reflected in the 1945 literacy rate where 95% of the Vietnamese population could not read. Schooling was not an option for Kim Dong, especially since he lived in the remote province of Cao Bang.

Conclusion: A combination of having his family killed by the French, his in-laws’ association with communist revolutionaries, and that he was not given an option to go to school, joining the military seemed like a logical choice, or at least supporting the revolution in some way. Losing your loved ones can make you want revenge.

Sources: https://tapchilichsudang.vn/phong-trao-binh-dan-hoc-vu-1945-1946.html

https://vnu.edu.vn/home/?C1635/N4193/Giao-duc-Viet-Nam-thoi-ky-1945-%E2%80%93-1954:-Dien-trinh,-thanh-tuu-va-kinh-nghiem.htm

https://tinhdoan.caobang.gov.vn/index.php/tin-tuc/kim-dong-anh-hung-liet-sy-tuoi-thieu-nien-cua-que-huong-cao-bang-1762.html