So junior AR does the ball and afterall as referee it not how you want it so you end up doing it anyway!
Or two mins in its clear you have to change ball, are you really going to point at the junior AR and go, sorry lads that was him?
your the ref, your ball.
If the junior AR makes sure the balls are at 11 psi, which is kind of the accepted standard where I am, there is no issue. Was also the same on the Conference, 4th official sorts the balls out whilst the other three are out warming up.