For the World Health Organization, the use of masks for children under 5 years of age would not be necessary, and in the case of the age group from 6 to 11 years of age it would only be advisable to use them depending on the level of risk they run.
This goes hand in hand with the recommendation that an adult should supervise the proper use of the mask by minors, in case there is a risk of contagion, such as in schools or near elderly people, among others.
In addition to the conditions described above, there is a note about the need to use a mask that is implicit in immunosuppressed children, patients with cystic fribrosis or cancer, among others.
Link to the WHO guide for the use of masks:

https://www.who.int/es/emergencies/diseases/novel-coronavirus-2019/advice-for-public/when-and-how-to-use-masks