Please Help

hi all im not the child of an addict but i despartely need some advice on how best to support 2 little girls (10 & 7) who are children of addicts, there mum dad & step dad are all herion addicts & have been for over 20 years im a gd friend of thier mum but have little understanding on what it is like to grow up in that environment the girls stay with me every other weekend from fri - sun i know they are both very angry & hurting & im sure a lot more goes on at home that im aware of , i want to help them understand that its not thier fault but dont know how best to go about it if any of you can help me understand what it is like from a childs prospective i would greatly appreciate it & i know the girls would to 

Thanks x