Is it legal for a minor to be at a club that allows smoking?

I do not know the answer to your question. You did not identify your state it varies from state to state. Also, some cities and/or counties have a more restricted law than does their state, so it may also depend on which city or county the bar is located in. I think GA has a no minors allowed in a smoking bar law, but I also think that there is an employee exception.