I've seen great kids classes with students younger than that (5 and up), sometimes with 20+ students, but I think having a firm and confident teaching style is critical to making them work well, as is a lot of energy! As you've said there's lots of good stuff on here about that aspect from folks with loads of direct experience.
I believe there was a large study in Japan about the effects of joint techniques on children which has informed the policy of the Aikikai affiliated societies in the UK but I haven't been able to dig it up. The BAB (umbrella body) has a whole suite of policy and guidance documentation on safeguarding, child protection, privacy, etc. but obviously these are in the context of UK rather than US law. They might be a useful starting point though.
We're very cautious about joint techniques in our society, using things like pressure inside the elbow instead of applying kote gaeshi at the wrist, and allowing uke to come onto techniques in order to manage the level of its application themselves. IME safety issues can often play out the other way around in mixed groups, with kids loving to pile on nikyo etc. whilst seeing adults as essentially bullet proof, (my youngest is a nightmare for this). I know a godan who was off the mat for 3 months through a 12 y.old girl putting sankyo on him and taking out all his soft tissue from wrist to shoulder, by his own admission his own fault for not respecting her and the technique sufficiently to be ready to respond.
Another key issue in mixed groups is managing mat space of course, with all adults needing to be especially conscious of where the children are and trying to keep them to the edge of the mat as much as possible since the really serious injuries all seem to come from people landing on each other.
Standard caveat: Please take all this with a fair pinch of salt as I'm a complete novice and someone better informed will be along shortly.