If i were you i would keep babies with mum as long as possible,feed mum kitten food(good quality)this will help build her up,also you could give her a kitten milk substitute,i have kittens here that are 4 months old,and 9 weeks old and still with mum,both mums were skinny but are now filled out nicley despite still feeding.I used to seperate at 8 weeks but now know its to early,so i intend to keep untl they are at least 12 weeks old.The longer they are with mum the better they will be for it.I feed royal canin kitten 34 to all my cats,when kitts are tiny they have royal canin baby cat.