The whole point of my asking this question was because mini-tennis removes the issue of high balls, at least through Red and Orange. Interestingly, when I was doing some research last night I found that Red (8&u) the ball never bounced above shoulder height for the contact, Orange (9&u) the same, ball might bounce above but never required a contact above shoulder.
Green (10&u) and Yellow (11+) however, were a slightly different story - seems to me the development of the child physically doesn't stay ahead of the compression of the ball (speaking generally) so they have to deal with higher bouncing balls for their height at Green and Yellow, relative to their height at Red and Orange.
My thoughts on the grip are therefore confirmed (I think) - eastern (knuckle on 1) to begin and allowing it to slide around to knuckle on 1/8 or 8 as required by the move to green/yellow.
I may be average... but my average is still ****ing awesome