Two words - coffee slave.
It's a place of employment, so put her to work. No need to make your own coffee when you've got a coffee slave in the office. Make sure she knows everyone's preferences (Bob = white with 2, Betty, decaf with 2, etc).
But seriously folks -
I would have a word to the appropriate workplace health and safety body in your jurisdiction. Having an unsupervised 8yo child in the office could be an occupational hazard, and may also breach regulations that govern worksite childcare.
The duties of the staff in the office also need clarification, as it is not likely that adhoc childcare comes under the aegis of "other duties as directed". It is unlikely that the staff would have been advised that childcare would have been included in their duties when they commenced employment, so such clarification would be necessary. This is especially important if childcare is regulated in your jurisdiction - such as a designated place for childcare, appropriate supervision by staff with appropriate qualifications, etc. While this is not as important for 8yo children - who can be reasonably self-sufficient - this does set a precedent in the office, and soon it could be extended to other younger children.
I like the idea of having the boss sign a waiver that indemnifies the staff in the event of accident to the child. Make sure it is drafted by a lawyer, who would know the appropriate language to put in the waiver. The waiver should assign to the boss all responsibility for accidents to the child. It should also indemnify the staff for such accidents. Make sure the boss signs it before allowing the child onto the premises again.