Children from 6 to 12 years of age accompanied by a person of 12 years of age or older carrying a valid travel document, on condition that they travel after 09:00 on weekdays (maximum 4 children), do travel free on the train in Belgium
Children under 12 years of age with a ‘large family reduction card’ travel free with the train. This is only valid in second class.
On the Bus, Flanders (De Lijn): Children do not travel free over 6 years old https://www.delijn.be/nl/