I've stayed at both with my kids: at the Poly when they were 1 and 3 and at the GF when they were 2 and 4.
As a grown-up person, I liked the GF better than the Poly. I like the light & airy feeling of the buildings and the pretty gardens and fountains. From a kid's perspective, the pool is really good for young kids, too, and the counter-service restaurant is nice, because you can sit outside to eat and get a view of the castle.
My 4 year old preferred the GF to the Poly, BUT, my 2 year old was in all of his 2 year old tantrum glory, and I felt very self-conscious about his behavior in the formal setting of the GF. There are fewer children, and less understanding adults (I think from the conferences that take place at the GF).
So with kids, I think the Poly is a more family friendly (tolerant) atmosphere. It's not as nice (IMO), but there are more young children running around. The pool is great for kids, but there is no actual playground (just a water splash zone). If you're kids are the low-key, quiet type (like my older son), then they'll enjoy the GF. If they are more 'free-spirits', like my younger son, then the Poly may be for you.
