Backing up throws in the infield

Can someone tell me the proper instruction for 8-10 year old boys on backing up first and third base on infield plays. Should the catcher be taught to run down the line at first base and backup the first baseman on a routine play to first OR is it the job of the right fielder to come in and back up the throw to first base. The same question applies for throws to third base. At this age, I can't imagine the catcher sprinting down to first and third on every routine infield ground ball.

Thanks for your help