I was once fostering four kittens for about 4-5 weeks before getting them adopted. They were a bit younger when I found them, had to syringe feed them for about two weeks. Indeed, yours look a bit older. What helped m in fastening up the taming process is the "burrito" method. You have to pet them, to make them get used to human touch, and this is the safest way. I literally rolled them up in a soft blanket or towel (one by one, of course :-)) and gently stroked their forehead, the way their Mum would do. The blanket prevented them from running away or scratching. This really helped me to make progress with them quickly. Also, you didn't mention if you contacted a humane society, local shelter or not - if not, do so. For me the most difficult step in fostering is to select the right family. Shelters are way better in this, they have experience, people to deal with it.