Well, I think it depends on the child. If the kid is using that time to progress his talent or is genuinely refreshed by it I see no issue. However, using this method as a one size fits all I air on the side of caution. Depending on how its applied those who finish work quickly can get bored waiting for others, and some who are slow can get anxious seeing those finish quickly.

Personally any free time I enjoyed progressing my own independent knowledge outside of school.