You should try to avoide having a pitcher catch or a catcher pitch because the arm motions are so different. If you throw correctly as a catcher you are going to have a short / compact motion that doesn't really circle down by the hip. To take a kid who throws like that then put them on the mound creates problems. If they try to do the whole arm circle like a pitcher it probably messes up both throws OR they continue to throw like a catcher from the mound and put too much stress on their arm.
One thing that helps maintain arm strength is working on their fundamentals about every other / third day. Most catchers never work on their footwork or throwing motion during the season or before a game. When they get tired they end up letting their mechanics slip and their arm suffers.
Another thing is keep them in shape. They need to run close to what pitchers run.
Plus these are kids and not men - give them a day off every once in a while.