When working with children (especially 7 and below) it can be vital to their memory retention to take a break every thirty minutes.
I have had great success with my younger students who become stir crazy after half an hour of reading by leaving the study are and going outside or in a space where we wont bother others and doing some physical activities.
Since time is a concern it is important to only do this for ten minutes or so.
Sometimes we run and play tag, or we will do some jumping jacks, or just do some silly dancing.
When the student returns they are feeling a little more refreshed, lighthearted, and ready to continue.
That being said, it is very important to make it clear that the activity is is only supposed to be for a few minutes then it's right back to studying.
I hope this helps!