I have two sisters who are both us citizens, and now live in guatemala what do they need to come back to us?

they been in guatemala since they were small 4 yrs and now they are 16 yrs, 14yrs what do they need to come back, just a pport and birthcertificate, or since they are underage do they need some kind of special doent either from guatemala or us ? and can they travel on the airplane alone thanks