Hi,
I weaned my pups with a raw meat diet, but I don't follow BARF. It is similar.
I gave my pups chicken wings and lamb ribs to suck on from the age of 3 weeks, it was just to give them a taste (suck!) of what was to come. I officially weaned them at 4 weeks old.
After about 1/2 a week I offered them a teaspoon of raw meat each to see what they did, I was also giving a communal bowl of milk in the morning and a bowl of milky cereal at dinner time, both of which contain natural calcium. I then introduced a teaspoon of meat at about 4pm and another teaspoon at about 8pm. I didn't offer bones as I wanted them to get used to the new tastes first, but their mother Penny was excellent and she began to regurgitate food for them with bones in from 4 weeks so I just watched them eat it. Obviously if the chunks were too big I'd cut them with scissors (Yuck) and if the bones were too big I took them out. At about 6-7 weeks I was offering full chcken wings and chicken thigh bones and they were fine. I did collect egg shells and crushed them to a fine powder and sprinkled them onto their food daily to make sure they had enough calcium. By the time they were 8 weeks the amount of meat I gave was up to a heaped dessertspoonful twice a day and I continued with the milk starter and milky cereal aswell. They could eat a chicken wing also by then, but Penny was still reguritating too.
I always watched them with any food I gave them and used to check their poops to to see colour and consistency. They were generally dark brown and formed which I was happy with, if they were looser I'd give wings if constipated hold back on bones that night.