The level of the charge will depend on the amount taken.
The 18 year old will be charged as an adult and the minor as a juvenile. It does not matter their relationship or the difference in their age.
I will assume that the value of the item was over $50 (if under $50, it is a class C misdemeanor) and under $1500. The adult is looking at potential jail time of up to 1 year, although they may be eligible to receive deferred adjudication. Because of their immigration status, it is vitally important that they have a good lawyer to provide them with representation.
The minor will be charged in juvenile court and if they have no priors, they will likely receive a deferred prosecution which will help keep their record clean.