Well I haven't stayed at a value resort yet (staying at the value all star sports August 2013 though). We have stayed at garden room at poly summer 2010, and MK view club level summer 2011. From what you say, being 3 adults and one 5 year old, I think a Poly stay would work out very nicely. The rooms are very big at the Poly. You can check out http://www.tikimanpages.com
which is a website with information on the Poly. It will have information like which longhouses have the biggest rooms for the same money as all the others, pros and cons to all the longhouses, and benefits to booking a club level room (if that's in your budget). It also has information on travel times from the Poly to every disney park. I can go on and on but....... you just have to check it out.