[bellyfullofbac]

But the random teenagers (my guess is it'd be more like twenty-somethings) would have the locals' guidance and supervision, surely?

And it would be a lot more efficient for the locals to use the manpower properly (with supervision) rather than letting them "play" and then having to tear down what they built.

At the worst case, the helpers could just be made to carry bricks and wooden planks from 1 spot to the other, and they'd still feel that they contributed.