When do boys and girls stop playing together in your communities? What types of play are allowed for older, under bar/bas mitzvah boys and girls? How do you explain to children that they should not be playing together when it is deemed not appropriate?
At age 3 by some and 6-7 by others. The relevant source (don't have where right now but have looked it up in the past and used it is not to "sit a tinok with a tinoket lest they become accustomed".
In Chabad and also on our DL yishuv there are no organized mixed activities after age 3 and the kids seem to just get it.
Any kosher activity (sports, board games, regular kids outside activities are all allowed by us). So some kids play music or draw. My ds was ping pong champ and nearly went into swimming competition. That we nixed because it would interfere too much with his learning. He is not the type to handle both learning and intensive sport training.
As to other kids, a girl once came and said "I came to see Rachmiel" and I answered "No you didn't". They are good friends of ours and the mother explained. They BTW had a real problem with this girl because she became very attached to a boy who came over to play and when she got to school she had a hard time reconnecting to the girls, none of whom played with boys.
Don't boys and girls naturally stop playing with each other in early elementary grades, if not before? According to the kids around here, boys are annoying and girls are yucky , depending on who you ask.