hard workers but not so smart

I am not a manager or anything, I am just my boss's able helper. I have a bunch of people reporting to me. However, if I don't like someone, I can ask my boss to let him/her go.

I have no problem suggesting to let someone go if that person is lazy and dumb. I just don't have the courage to let a hard working but dumb person go. The guilt will kill me.
Teaching these people is hard work on my end. I would rather just do the tasks and let them rest. It saves time and money that way, overall.
I can't give them anything difficult, because I will end up fixing them anyways.

Any suggestions?