We are also a family of five. The kids were 3, 5 & 7 when we stayed at PO-Riverside. The price was right, the buses worked great and the pool was a lot of fun. The room was small, especially when the trundle bed was pulled out, but we were in the parks most of the time. I do remember one late night struggling to get three sleeping kids off the bus and to our room.
It was a great place to go for a first time family trip.
Of course, we couldn't stay away so we went again last year (kids were 4, 6, 8) and we stayed at the Polynesian. Ah, was that nice! Much larger room with a day bed, queen-sized beds and a balcony. Quick access to the monorail for MK and TTC for Epcot. Beach area and zero-entry pool. We did miss the food court and the hot tub at POR, and we did pay more for the deluxe but it was well worth it.
If you are on a budget, go for PO-R. If not, stay at one of the deluxes!