It's always hardest with our own children when we do home daycare.

Firstly, you did the right thing by removing her from the group, and reading a book about angry feelings. That's awesome! Don't feel badly or uncomfortable around dcb's parents. Your child is part of the daycare and is also learning social rules and boundaries, just like the other children in your care. You did not show any favoritism and you explained the outcome to dcb's parents.

Some children, like adults, don't mesh well together. Dcb and your daughter could just have "outgrown" their friendship and now view each other as rivals of sorts. I would up the structured activities. Break them up in groups of 2 when pt dcg is attending. So when doing crafts, or puzzle or whatever activity you have set up, allow them in their groups and then rotate groups. Praise your daughter when you see her interacting well with dcb (and the other girls).

I would also keep a closer eye on dd and dcb. Maybe hang back and just watch them closer during freeplay so when another tug of war occurs over a toy (and it will) you can intervene before it gets to heated. I'm guessing your daughter is 3?