When one has nothing to do, one is more likely to get into trouble. When I'm off from school, my grandmother is always trying to get me out of the house, while reminding me that idle hands are the devil's workshop.
The workshop of Santa Claus, where Santa or his elves are said to make the toys that he delivers on Christmas. I'm sure the elves in Santa's workshop are making the toys you'll get for Christmas right now, honey.
Prov. People who have nothing worthwhile to think about will usually think of something bad to do. We need to figure out something constructive for Tom to do in the afternoons after school. An idle brain is the devil's workshop.