Given that generally, listening to music is prohibited during the Three Weeks, may an adult camp counsellor dance with below bar / bat mitzvah campers, for the entertainment of the campers and to contribute to an energetic, joyful environment?
See this article in this year's Agudat Yisra'el pamphlet, which was reviewed by R. Shmuel Fuerst. On p. 13 it says that music that is not for the sake for enjoyment may be heard during the Three weeks. Among those exceptions is a teacher dancing with young children.
Note that by this phrasing, it implies that not only may a teacher dance with the children, because this is his / her profession and it is for instruction, but, s/he may also listen to the music because it is being used for instruction, not for pleasure. I'm inferring from this as well as the other items in the list that others who happen to be in the room and are not involved with the instruction may also listen to this music. It seems similar to the music played in elevators or hotel lobbies which is permissible.