Under the age of 6, they don’t have to pay in the Brussels Region with the STIB/MIVB network (tram, metro and bus). It is enough for child to be accompanied by an adult in possession of a ticket in good standing. For children between the ages of 6 and 12, parents must apply for a "J" subscription (free of charge with 5 euro deposit) which remains valid until the child's 12th birthday.
In Wallonia, children under the age of 12 travel free by bus on the TEC network. In Flanders, children under 6 travel free of charge on the De Lijn network.
For the train: children under the age of 6 travel free when accompanied by an adult with a valid ticket (any paid traveler can take up 4 children for free).
For more information: https://www.belgium.be/en/mobility/public_transport