In your shoes:
I'd give the "I can no longer meet your child's needs..... last day of care to be on.... Thank you for choosing me....... Wishing you much success in your future....." blah, blah termination since telling them their kid is acting out violently and a risk to your livelihood does not go as smoothly as one would expect....
Then I'd tell DCD Thank you for being patient and trusting my judgment while I attempted to work with child a's needs, but after much effort I have made the decision to let them go for the benefit of the group.