If, for example, I am talking to (or about) a group of children and adults, would I normally use honorifics that were appropriate for the highest-status members of the group?
Assuming that is the case, would it ever be appropriate to 'talk down' to a group consisting mainly of children, but including some adults? For example, would adults present in a supervisory role understand that if the group as a whole were addressed without honorifics, that it was only the children being addressed?
It's not an easy question to answer. But I will try.
I am talking to (or about) a group of children and adults, would I normally use honorifics that were appropriate for the highest-status members of the group?
No. It depends. Usually '하십시오체' will be used when adults are addressed. If there are 10 students and 20 parents in a class room, there is no way a teacher would talk down to any of the groups even if only the students are addressed. It is normal (not uncommon) for a teacher to use '해요체' to students. Whether the teacher's words are directed to the students or parents can only be understood from the context.
Would adults present in a supervisory role understand that if the group as a whole were addressed without honorifics, that it was only the children being addressed?
Again, it depends. As mentioned above, a situation where a whole group of adults and children are addressed without using the honorific is difficult to imagine.
