"The children start school in the autumn."
Translation:Mae'r plant yn dechrau'r ysgol yn yr hydref.
For the same reason that we say mynd i'r ysgol, mynd i'r gwaith, mynd i'r gwely and so on when in English we say 'going to school, going to work, going to bed. For places we go to habitually, Welsh uses the definite article in i'r... where English does not.
In this example, the children will be going to their school regularly, so the definite article is used.
See the notes for the section 'Simple Future'.