This would depend a lot on the situation. One of my first things that I would think of would be to look for the child with a family member who did not have custody of him. That is, I would check to see if there was a divorced father who might have taken the child.
Outside of that, I would look in places where the child likes to play. The boy might have wandered off and gone to play at a playground or a park. He might have gone to an arcade if there is one around nearby.
Of course, it is possible that the child was kidnapped by a stranger, in which case there is no way to know where to look. If we are not assuming that, we should look for him in places where he would play or with someone who had a reason to take him.