Raz,

To use any kind of 'random' thing in powerpoint - requires the use of
something called VBA (a programming language)

Or you could 'fake it' by having all the slides ready made, then by entering
a slide number on the keyboard, and pressing the ENTER ley, you could jump
to any particular childs name slide.

cheers
TAJ Simmons

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As Taj suggested, this is a job for VBA (to be done truly randomly)

What I would do, if it was me,  would be to have all the students pictures
on the screen.  When you press a button (to activate a macro), they would
get an outline around a random picture for about .2 seconds,  in random
order and for a random number of times.  This would keep the selection
fairly quick (between 2 and 5 seconds) and still make it completely random.
You could also make it so you could force the spin to end on a student, but
that wouldn't be very fair.

Are you familiar with VBA?  What version of PowerPoint are you using?  Is
VBA enabled on your machine?  Do you have admin rights?

As the others said vba will give random actions. However you can simulate
randomness without vba. I've uploaded a simple demo of this here
http://www.PPTAlchemy.co.uk/testing.html. Click anywhere (except restart) to
get a 'random' selection

You might also like to go to:
http://www.pptalchemy.co.uk/powerpoint_hints_and_tips_tutorials.html
and scroll down to Randomness hints tips and Tutorials