They can fire you for working too much overtime, even if you do not report it and ask to be paid for it. You are an employee. Because they have given you a managerial job, do not fall into the trap of thinking the job (or getting the job done right) is more important to you than the pay. It isn't, because you are not the owner, stockholder, person who is responsible for the overall well being of the company.
And don't think that you have to work more than you get paid for because without you doing that, it's impossible to get the job done. It is only a job, for you. They are paying you for xx amount of time. If you can't do what needs to be done in this amount of time, inform them. Ask them what they want you to do. Then quit worrying about it. It is after all, their problem, NOT yours.
If you legitimately can't "get it all done" in the hours given you, then it is up to the owner/manager/head honcho to decide if they want you to work overtime, want your employees to work overtime, or if they want to hire another employee or even if they want to fire you and try to find someone else who can get more done.
In many jobs, working extra time, unauthorized overtime, is one of the most egregious of offenses. Managers who give out too much overtime when they've been told not to get fired. Being there, even if you are not on the clock is accumulating time that they might actually have to pay you for someday. This includes staying late, working through your lunches and breaks, working far into the night or taking work home. Not only could they fire you for working too much overtime, but you would be considered insubordinate if you'd been told not to work overtime, and you probably wouldn't be able to draw unemployment benefits after they'd fired you. It would probably be considered a valid misconduct reason to terminate. Same goes for if you are salaried but you are giving your employees overtime when you've been told not to. It could very well get you fired without unemployment to back you up while looking for another job.