We are planning a trip to the area in September, 2018 and would appreciate any recommendations for a family of 6 adults.
You may get more responses if you post your question in the general "Italy" forum. Not everyone sees the "Reviews" section, which is used for reports on places already visited.
I found Donna Eleonora Residence (www.donnaeleonora.net) to be perfect for our visit to Matera. It is a B&B right at the edge of the Sassi. It is actually a flat consisting of 3 apartments and a common area. Modern, clean, and friendly, helpful proprietors. Our room slept two and had an en suite bathroom. Not sure about the other units, but you could enquire. Reasonably priced.
We stayed here last year. Great location. Hotel San Domenico al Piano