On site, they could stay at Art of Animation, Port Orleans Riverside, Caribbean Beach Resort, Fort Wilderness. Or they could go Deluxe and stay in a standard room at the Poly or Wilderness Lodge, or a Studio villa at the Poly or Wilderness Lodge. Some 1 bedroom villas will sleep 5+baby as well.
I think 6 in a studio villa at Poly is pretty equivalent to two of the other options (CBR, POR), and gets a transport advantage. 6 in anything short of a 2BR villa or AoA is tight. It's about what trade-offs they want to make: time, transport, space, money.I would question staying at the Poly in a studio villa. We just had a stay in a studio villa as a family of 4 and we were a bit cramped. Plus you only have access to one bathroom. I would suggest a studio maybe at Kidani Village if you can rent points from a DVC member as you'll have 2 bathrooms there. It will make it a bit more manageable. You can also get two rooms at Poly that will work as well or see if you can rent a DVC 2 br villa.