If you prepare returns for them, it would be a return each. Remember if they do not owe then they are not required to file a return. If there income(s) were less than $11,474 each then the basic personal amount credit is more than enough to determine a zero tax liability.
While they may not be required to file a return, they would not get a refund if they were entitled unless they file. With TurboTax you can prepare returns, and are only required to pay if you decide to actually file. There is also a free version of the software...you might wish to look at this, particularly if the returns are not complicated. If you wished to prepare returns to see what if anything may come back to your sons you are welcome to do so with no obligation.