Is there a skull cap available for the kids? If so, you could add a noise node as displacement and get stubble that way.

If there's no skull cap (I've never really used the kids) you could select the necessary polygons with the grouping tool and make a new material zone -then apply the displacement to that.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.