The humanitarian assistance is meant for children, who have been living there with their mothers for more than a year already and still have no opportunity to return home.
I prefer to use here the PPC. But is it possible to use the PP (Present Perferct) here? Do I need to put comma before "who"?
Yes, both tenses would work here, with no real difference in meaning.
I would consider that who clause as a restrictive clause, so I wouldn't separate it with a comma.
I would put a comma after 'already', however.
Wear short sleeves! Support your right to bare arms!