I would suggest reduce their playing time, not using them as starters. I would also suggest that you talk to their parents and let the parents know what is going on and what you intend to do before you do it. This alone may solve or reduce the problem. I would like to know what the effects of continous exercise are on the body. There is a theme here, however.
Regular exercise is important to proper growth and development.
While daily exercise is not necessary for adequate benefits from exercise, it is also not necessarily bad. It depends on a variety of factors including the types of exercises being done and the intensity with which they are done. One note of importance regarding training effects in general. To produce an beneficial effect, the training must put a stress on the body. What is also necessary is recovery time from the stress that is produced.