There is NO valid reason to hit a child, to supposedly "teach" them. Outside of possibly a blocking exercise there is no conceivable reason to make any contact...
<<they train kids like they do adults.
What size and how old is your specific group?
<<my Sensei won't allow that kind of disipline.
That is not discipline, you describe abuse, and your instructor is dead right not to allow it! At 16 you are certainly able to help out with younger children. Discipline is NOT something you will be able to easily maintain... nor should you. They are NOT mini adults... they are c-h-i-l-d-r-e-n. I assume you have basic rules? Enforce them firmly & fairly. DO not try and be your teacher, be YOU, a 16yo person, with certain understandings and experiences... Do not pretend to be me.
Kids will try and see how far they can go with you, as the "new teacher", and will test you. They are little, young, kids. Be consistant, be firm, but stay in control... yelling is never a good teaching technique.
"...Actions have consequences. Correct actions have good consequences, incorrect actions other consequences. You are in charge of what happens to you... choose wisely class..."
Give them a warning perhaps? Tell them what reward you intend if things go WELL today... Regardless make certain as individuals they actually heard what you said... if they cross the line twice, you call them on it. If safety is the issue, do whatever is necessary to maintain it.
Young children have minds but need guidance literally. They understand many things but because they are children, cannot see the outcome of most of what they do, because they are kids!
Are you dealing with specific problems, or simply overwhelmed in general? If there are specific problems many can help with more details...