The answer to the question of what rules can young players avoid until later is it varies. There is no one correct answer. If your little one is enthusiastically determined to jump into the arena or sports, then the best thing to do is to speak to the coach for the team if the child is joining something already organized.
If you don’t have a coach and are considering starting up or organizing sports activities for young players, then get in touch with people who have experience doing it. Go ask questions of P.E. coaches in elementary schools or interview some day care providers if they are willing to give you some time.
The number of sports and all the different rules that are part of the sports world cannot be done justice in the amount of words to be found here, so it is recommended to consult people tied to the sport specifically.
Most rules were created to maintain two details. Safety first and fairness. If the rule does not pertain to the safety of the players or spectators and if the rule does not pertain to maintaining the fairness of the game then it may be left out.