Under 10's? It needs to be fun. You're not going to have a dojo of little angels. You're best bet is to try to keep it down to a low roar!
Like most kids, they actually like a routine. They want to know what's happening next (it helps with their concentration and engagement). Set and keep boundary's. They should know not to 'misbehave' before class, to stand quietly when bowing in (or whatever your 'start of the class' procedure is) and they should know that the first thing they start to do is warm ups. (I get them to run a circuit around the dojo then get them to jump, hop, skip, side walk, roll, crawl, etc etc - pretending to be animals is good too!)
Then you do your "technical bit", then a bit of practice, then a game. Rinse and repeat.
Give the kids a favourite game at the end (we play "sumo" with them). This is their reward for a good lesson.
As a martial arts teacher, the way you behave directly influences the kids (and the parents) so you must never lose your temper.
It's much better to reward good behaviour than to discipline bad behaviour. If you know "little Johnny" is a PITA then try to catch him doing something good, and reward him (try to get that in early). If he then starts to play up then tell him your disappointed 'cause he was doing so well (and the warning) and if he repeats the misdemeanor then he will sit out for 5 minutes. Then catch him doing something good again (real quick).
It's funny, but it does work! Everyone wants to be accepted, wants to be liked. Some kids have learnt that they can get attention from being bad. You need to show them that they can get better results by "just not being that bad".
Finally, at the end of the lesson, re-enforce the good things he did and say "see ya next week!". And start next week completely fresh with no hang ups or carry over from the previous week. If the kids comes back, it's 'cause he wants to come back so give him credit.