There can be many different reasons, and each child has a story to tell - sometimes a horrifying and heartbreaking story.
But it can be as simple as orphans living in poverty. The picture above shows a family with six children, looked after by the grandmother (I am not sure who the other two adults are - they are not the children's parents, who are dead.)
This old lady simply cannot cope - not only financially, but with handling six lively children, who - as in any family - can be rebellious and defiant.
And sometimes a child in such a family decides that they would be happier living independently on the street, and runs away to the nearest town.
What can be done?
Help and support can be given to the caring grandmother - such as instructions how to make a garden to grow food, and setting up savings and credit schemes. Enos Kyibibi, Director of the SKILL Centre, is involved in such support.
Likewise runaway children can sometimes be helped to return to their family, though this is not always appropriate, such as where there has been abuse.
Here is Enos outside the new SKILL Hostel.