First don't try to housetrain a 6 week old puppy, get a exercise pen where you can contain them and use the potty pads right now. They have no concept of bladder control and thier attention span is non exsistent. With mom around they would follow her out and potty, it helps alot. Save yourself some trouble and let them be puppies as they should be at this age. In the best scenario they would be with mom and littermates in a secured area at this age. You are fostering, so I have to believe something has happened to mom, that is sad.
It isn't easy taking pups in this young, so do the best you can and don't stress out with potty training. By all means do not separate the littermates until at least 8 weeks of age, if you can hold onto to them a week longer that would be better for the new owners. At 8 weeks I would start the housetraining without puppy pads, I keep my puppies crated, tethered to me and or close to the door. It becomes my priority and it pays off. Good luck fostering, it isn't easy.
A exercise pen, crates are your best helps. Also see the housetraining thread. viewtopic.php?f=6&t=6565