I think this is a good idea.
I know, personally, that I get more done in 15 minute chunks with a break in between, then I do if I worked at it for an hour.
Kids NEED that social interaction to become better members of a community.
At the school I am working at at the moment they do 'brain break', 'snack attack' or something similar. After being in class for nearly an hour, they stop, go outside and eat something healthy. It can be vege sticks, fruit, fruit yoghurt or half a sandwich. It cannot be sweets, chocolate or junk food. It gets the kids refocused and ready for learning again for the second hour of the session. This makes sense as it can sometimes be 3 - 4 hours between breakfast and lunch for these kids.
The class I was teaching for a full day on a Friday would have movement break half way through the second session as they need the short break. It's only 10 minutes where they run around, but it helps them focus.
Having seen the improvements in this class with ADD and ASD kids in it, I think it's a great idea.