Since it's the tone of LIS that is at question here. How about instead of TOS, have a cross over with Star Trek: The Animated Series, that was more "juvenile" friendly.
That's a common miconception. In fact, ST:TAS was intended and overtly promoted as the first Saturday morning animated series aimed at adults. From Lou Scheimer: Creating the Filmation Generation by Scheimer with Andy Mangels, p. 96-97:
So, yes, still accessible for kids, but challenging them to rise to a higher level, and offering adults something suitable for them as well. Note that TOS itself, though intended as adult science fiction, always had a strong following among children. So TAS essentially had the same target audience as TOS and was written to the same level, aside from toning down the sex and violence for the time slot. While other cartoon adaptations of live-action shows shifted the focus to teen sidekicks or added cute animal mascots or changed the shows to have more fanciful formats, TAS strove to be as authentic a continuation of TOS as possible, just with less kissing and killing and more exotic aliens and settings. (And bigger roles for Sulu and Uhura.)In June 1973, Norm [Prescott, TAS co-producer] was interviewed for a Newspaper Enterprise Association story about Star Trek, in which he said, "This is the first attempt to do an adult show in animation. Never before has an adult audience been challenged to watch a Saturday morning show. We feel it is a bold experiment." [TAS director] Hal Sutherland added, "The problem is that kids have not had a choice on Saturday morning. We're going to find out if they'll go for more sophistication."