Ah...but to have been taught appropriate play from the beginning allows the child to behave appropriately.
Yes, it most certainly does. That is why both forms of learning are necessary.
There are built in examples of conflict resolution in sports. The mentors also teach mature ways by the child seeing how they operate in the real world.
There are examples of cause and effect, rulebreaking and punishment, rule-adhearing and escape of punishment, practice and positive performance. However, sports do not always offer replacement behaviors (sometimes they do), most sports involve breaking a rule and being penalized, either in points, or in removal from the game. In fact, sports can foster aggressive attitudes and behaviors: Did you know that in basketball, if someone has the ball, and someone is guarding them, the person with the ball can pivot, jam their elbow into the defender's nose, break their nose, and a foul will be called on the person guarding?
Martial arts are particularly adept at teaching respect for one another. The children that had played the most with other children usually became great at compromise...of themselves. Giving of oneself more than what is fair to oneself is not conflict resolution...it is cowering.
I agree with the statement about martial arts--martial arts is also very good about providing replacement behaviors (I do not do this; I do this instead). I also agree that simply giving in is not conflict resolution; it's rolling over and being a doormat. However, effective conflict resolution tends to involve compromise on both sides, such that both people can agree to a solution. One cannot always insist on their own way, and it takes creative and intelligent problem-solving skills in order to find a solution where you are not "giving of oneself more than what is fair to oneself."
Now...playtime could be with children of morals their own age...but with something to teach. Again, sports. Yes, uncontrolled play is great for problem solving skill development....if they learn the correct lesson...if the group desires equality for individuals...if not...they are learning either domination or acquiescention.
Yes...through modeling...older...more morale...somehow better...children will learn to become better more efficiently.
You are assuming that there is a "correct" lesson to learn. Unfortunatly, with unsupervised, unstructure play, children sometimes learn that they can "get their way" by various maladaptive means (threatening, physical violence, lying, yelling, etc). However, in loosely supervised unstructured play, children can work out conflicts with scaffolded adult involvement--that is, if one person is getting out of hand, the adult steps in to say, "We don't (yell/bite/push/etc). Find another way to work this out." Thus, the children learn that their maladaptive attempts are inappropriate, but are put in a situation where they need to think critically and suppress impulsive behavior in order to find an appropriate solution. Of course, one would expect different levels of independence with this (and thus varying degrees of support should be provided) based upon the child's age an maturity; more adult support would be needed for a four year old than for a nine-year-old (I would estimate that the aforementioned scenario would be effective for children whose maturity is around the "third grade" level up through adolescence) . However, the adult should only be involved as needed to provide the appropriate supports. Otherwise, the child becomes dependent upon the adult to problem-solve for them, rather than learning to think the situation through and make appropriate decisions.