Like anything else in TMC, a "TALK" or "calling" or otherwise assignment is for the audience, class, members or public, but just as much, for the "called" missionary, teacher or speaker giving a talk.
The subject is generally decided upon given a combination of factors, ending with a bishop thinks you need to learn or grow your "testimony", + that bishop tells you to speak on something he thinks someone else, or the congregation should hear, or that an old man just spoke in general conference and now is in a magazine, and 'that' is the subject.
If kids [need to] give talks, these should be done in monthly presentations, or a relief/ change/ otion Sunday School class for child's development and practice of a child's speaking skills and confidence. Curious adults could visit the youth run class where they would call each other and listen to each other perform or speak.
WORSHIP time/ sacrament meeting should have decent adult spiritual conversations, speeches and subjects of interest and it once a month testicle-skipameal-circus should be eliminated and relegated to the 10 minutes in between classes, where one can bear their testimony all they want.