I don't teach martial arts, but I think some of the things I've picked up as a history teacher can help.
Start off heavy with discipline, you can always ease up if necessary. The reverse is not true.
It is possible to maintain discipline, entertain, and help kids learn at the same time. This is not a science, but an art that has to be developed. Easier said than done . . .
You don't teach a kid, you help him learn for himself.
Focus on need. Structure the class (and individual instruction) on what the kids need, not what you want to teach or they want to learn. Some kids may need more esteem building. Some kids need ethical lessons. Some kids need discipline. You can't do the physical side of aikido properly without the other stuff in place.
Respect is a two-way street. Don't demand it, try to earn it by being a positive role model.
Publicly correcting a kid may make you feel better temporarily, but you lose the kid. He won't respect you, so he's gone mentally and emotionally, and eventually physically. Private talks with kids focusing on the behavior (not the kid) will have better results. Also, when speaking to the kid, it helps if you turn the focus off the kid (I can't teach when some kids . . . or "the other students can't learn when some kids . . ." It also helps to make a "sandwich" out of the discussion- positive comment, constructive criticism, positive comment (Your ukemi is incredible, but you won't keep learning if you . . ., once you get past this negative behavior, you'll be a class leader).
I don't know if this helps . . .