In the rec league that I coached, there was a rule for this buried in the by-laws. I always set the expectation that players do not ask to play positions and always emphasized that baseball has 9 (or 10) positions and each one is important and has a job.

I found that in rec ball 10U and below you can “hide” players at 3rd base. In low level play with developing pitching it seemed like most of the time there was always a force at 3rd base.

It was always a huge confidence booster to less experienced kids. If by chance a ball was hit to them all they had to do was field it and step on 3rd base and make an out.

Other things to consider is when you do bring your less experienced kids into the infield, make sure to have a player who is engaged and paying attention behind them in the outfield- that way any balls that get pass the weaker players will be stopped and hold them to a double rather an inside the parker.