It's been a while and I could be wrong but I think they start eating moist food around week 6 or 7. That's why you normally don't send them away before 8 weeks.
The pup I had was so weak I think I started her at 4 weeks.
One day I saw she couldn't hold her head. She was going down really fast. We got her to feed on mom first before she went to feed the others but I also got some food on my finger and put it in her mouth.
She'd spit a lot but the little she got in helped I'm sure. I remember having her with me most of the time because the other pups would throw her around, poor thing.
The vet could not see anything wrong with her and I never found out why she was so weak but we saved her
She got to be a healthy dog but it was iffy for a while.