definetly a four stroke, the power curve is much more forgiving and the low end grunt makes learning the clutch uber easy.
a bike can definetely be too big, but not really to small to learn on. i had a blast even when i was older tooling around on smaller bikes, i helped beat an xr100 into the ground and loved every minute of it.
you can start off with one, let them rotate and see if they really like it. if there serious, buy more.