As a general rule the USSF is not a big fan of coaches being involved as players in the sessions. Their contention is that coaches playing in the sessions really manipulate the way the players act.
Younger players want to please the coach, so they will try to get the ball to you at all times, even when this is not the best decision.
You don’t want to be involved in this way. There are some advantages however, to having coaches who have played at a high level be involved in a session.
Players learn best by imitation so if you are able to model good technique it can be beneficial to your players.
However, you don’t want to be the one dictating play; you should be serving a more subservient role. It is important for the kids to be able to see what a good soccer player does on the pitch. Set a good example.
Having a coach as a facilitator can help younger players experience possession and rhythm. You know if you are doing a proper job if the collective quality of play improves when you are a part of the session.
More importantly, is the quality of play sustained when you are no longer involved?
Having a coach as a neutral player helps accomplish this.
If you do step in, you have to be honest with yourself as to whether you are good enough and fit enough to play with the kids that you are coaching.
You don’t want to be the cause of the breakdown. If your playing adds to the intensity and quality of the game, then the kids can learn a lot from observation. Older boys especially like to see that you can practice what you preach.
You certainly don’t want to play in every session, but jumping in from time to time can be valuable. Most of your time should be spent coaching.
Hopefully the kids will respect your knowledge of the game even if physically you can’t do it any more. But you never want to put yourself in a position where the kids lose respect.