Hi Brooke! My husband & I have been fostering for many years... We usually start our babies on what I call 'kitten mash', which is formula mixed with a little canned kitten chicken food (I use an emulsifying blender)... We do this as soon as their front teeth start to pop (about 3-4 weeks). I should mention that at 3 weeks old I put a small (short sides) aluminum pan in their cage with potty sand. At the time we start feeding kitten mash, we add dry food in a small bowl a few days later and a water bowl too. Some will naturally supplement their food in a week or so. Once they all have gotten their bellies back in shape (any change in food will give them soft stools for a few days), then I sit on the floor with a bottle & a plate of the kitten mash. I bring lots of hand towels & a wet wash cloth or two also. Some will go right to the food, others will take a few days (thus the bottle)... It takes a few litters to figure out a schedule that works for you & the babies. What I described above is what has worked for us...
As for the diarrhea, all babies will get a touch of diarrhea when you change the food, so you will want to mix in a little at a time, but it is not uncommon for this to happen...
You are doing a great job! Keep it up!
#PetHealthandNutrition