NO question--Poly all the way.
The pool has zero depth entry (a huge plus for little ones). CR's does not, and neither does the WL.
The theming is very kid friendly with tiki statues, music throughout the grounds, and many "Alohas" from CMs.
Food options are good. My kids absolutely love Kona believe it or not, and 'Ohana is a blast. You're only a short monorail ride away from Chef Mickey's, too.
Rooms have been recently renovated, and they're among the largest on Disney property. WL's are very small compared to the Poly's.
Transportation is the best. You can walk to the TTC and catch the monorail for Epcot without having to switch monorail lines. That's a great perk neither the CR or GF offer. You can also take a boat to MK if you wish (in addition to the monorail).
GF and CR are both having some construction right now, and depending on when you go, that could affect your plans as well.
We stayed there last year, and we'll be there in just 6 days. My kids and DH refuse, and I mean refuse, to stay elsewhere.