Children up to 12 years of age are not allowed to travel unaccompanied.
However, as of the age of 5, children can travel as an "unaccompanied minor". In that case, the airline takes care of escorting the child against payment.
The child should be taken to Brussels Airport by a parent or guardian and similarly collected at the other end.
During their flight the child will be under the permanent supervision of airport personnel and cabin crew.
Attention: not all airlines provide this service!
For obvious reasons, rigorous procedures apply, including a thorough identity control.
Check with your airline or travel agent.