Hi Sheila,
Officially, Disney's complimentary transportation is available one hour prior to Disney Theme Park opening time up to one hour after Disney Theme Park closing time.
That said, the folks at Disney are aware that many Guests staying at Disney Resort hotels will have made early morning breakfast reservations and I can honestly say that my family have never had any problems using Disney's complimentary transportation to get us to them.
I would suggest that you be at the bus stop at your Resort hotel at least one hour prior to your dining reservation. Catch a bus to the Magic Kingdom and then from there, take the monorail to Disney's Polynesian Resort.
If you have any concerns at all, I would recommend that you stop by and explain the situation to a Cast Member at Guest Services at your Resort hotel the day before your reservation for confirmation that buses will be running early enough for you to get there in time.
Wishing you a magical trip filled with memories which will last a lifetime!