I think they pass for 6 weeks too. Have they been vacc'd or dewormed recently?
It's been a few years since our last litter, but I believe by six weeks, the pups were eating quite a bit of crushed/soaked kibble meals--we usually soaked in Esbilac--with dry kibble as a free feed option if their teeth were in. We prefer to let them wean naturally, so they were getting milk even at eight weeks when they could all manage to stop Mom for a quick nurse. By eight weeks we quit soaking in Esbilac and just mix in canned food if they aren't eating enough of dry. I don't think the timeline is too important, though--so long as they are drinking and eating enough, their development should be fine.
The more stimulation for development, the better.
Variety of toys, obstacles, new things check out (boxes are all sorts of fun!) every day. You can start rewarding good behavior as well. Lots of fun and quiet time with humans, getting them used to everyday household activities and appliances, getting them used to grooming (nails!), etc.... all that should keep you plenty busy! (By the way, the crate is a great place for them to be sleeping--should help with crate training later.)
Outside time is good--even if for just short periods of time, it being cold. Not knowing your property's virus history or their health history, I would probably keep them on easily sanitized surfaces, or if you have none available, put a tarp under an ex-pen.
Have fun! Puppies are a lot of work but well worth it. And that is so great of you to be fostering them.