I'm a teacher-- I do not like school age children at conferences at all because I don't think everything we say is appropriate for their ears. I want to be able to tell a parent that her child's reading level is such and such, whereas the rest of the class is such and such, and why we are taking steps to remediate this, without the child hearing. I don't want to discuss possible evaluations in front of a child. I don't want to discuss behavior issues and motivation and things going on at home/school that may be affecting each other in front of the child.
Kids younger than school age are hard because we're on a really tight schedule and there is SO much we want to talk about and hear from the parent that any distractions take time away from that. Depending on the number of kids in my class, I have between 10-15 minutes to meet with each parent, and there is just so much information the school needs me to give before I can even talk specifics. If even 3-5 of those minutes are spent aquainting a little one to the room and helping them, then that's 3-5 less minutes we can talk.