How to better pace activities

Question

I am working as a volunteer, teaching children programming once a week.

The attendence is somewhat irregular (and the children are a bit restless :P) so, instead of more traditional lectures, we try to give the children self-paced activities at the computer.

This class is goes on for 1hour 20mins, in which we usually have the students trying to complete a series of "programming challenges" aimed at teaching a particular concept.

Often, however, children get tired in mid activity. What can we do to re-energize the children? How long is it resonable to expect an activity to hold a child's attention?

(ages: 11 up to 15. But mostly 11 and 12)

Answer 1

I've seen in a couple places that children tend to have attention spans of under 10 minutes, while teenagers and adults may have attention spans of up to 20 minutes (example).

At least at the community college level, I try to frame all by presentations in 30 minute blocks, with about 20 minutes of me lecturing/presenting, and about 10 minutes of students doing exercises on their own (with me circulating with checks and advice).

So that may not be exactly what you want in your situation. But if you can find some way of switching between different contexts every 20 minutes or so (maybe lecture vs. exercise, or individual vs. group work, or something), that might help.