Zaphod, I am not giving practical advice, I am starting with an “ideal” scenario so that we can then tailor practical advise to that ideal, rather than to the “usual”.
See how stereotypes work: you say that kids in this ideal scenario have less social skills. I contend that kids who interact with more mature kids/adults will have better social skills. This is a well-known fact, for example, that younger siblings are better talkers due to interaction with older ones.
So, in regards to your concerns, if you want to form such groups, invite best behaving older kids to pair with a larger group of younger ones, do not invite the worse behaving older ones.