Minatsuki is still missing, presumably dead. That could have been the staff's target, since she was also not an adult.

[From where he is, Ciel regards Bigby for a bit, then looks back at Elizabeth.] I don't think Franziska would go down so easily based on what we had seen of her. In Bigby's altercation, he received injuries.

Whoever killed Franziska likely will also have injuries, ones fresher than those from the glass breaking. [He points to the wound on his brow. A bit irritated because of the eyepatch, but healing.]