i agree with mrss go to the presidency that is what they are for. this is their problem too. and if you have a big enough ward where the presedency isn't having to teach that is what they have to do is help the teachers.
i was the primary pres. with one councilor and just enough children to make it difficult, but not enough to have different age group classes. we had everyone, 11 yrs to 18months in the one group. we had several under eight who i don't think ever had any dicipline. one sunday i cued up the cd player to "reverance is more than just quietly sitting", i hope you know which one i am talking about. anyway, everytime i couldn't talk because of their noise i would start it up and start singing. when they got confused as to what was going on and got quiet it i would stop the cd and continue. after awhile some figured out what was going on and started singing with me (i can't sing mind you). now i will admit that the first couple of weeks we didn't hardly touch the lesson, but it did work.
another possibility is to bring in some small cups, with their names on them, and some marbles or something tangable. when you see good behavior stop thank the child for the specific behavior put a marble in their cup and then continue. don't correct the bad unless it is really bad only acknowledge the good. keep a chart of how many they earn each week (it could be on the wall or in your roll book) and at the end of each month give a reward. i agree with mrss again on no candy, stickers or something (check the dollar stores if you have one they are great for that). don't put a number on how many marbles they must earn, just if they earned more than they did the week before. that way you are rewarding their improvement not a quota. i hope that makes since. the goal is to give as much constant positive feedback as possible. we all like to be told good job. don't be shy with the reward, if john has been wandering the entire class time don't tell him to sit tell everyone who is sitting thank you for sitting in your chair and then give their cup a marble. john will see and hear this. when he does finally sit give him a marble within the first 30 sec. or less. be quick. even if he only gets on marble this month give him a reward anyway, however next month he must earn at least two to get the reward. must show improvement. tell them that they are working to show personal improvement, so they know what the expectation is to earn the reward.
i hope that made since, these things have worked for me. but you will have to find what will work for you. pray, heavenly father will guide you, he knows what to do, they are his children.
good luck
ALmom