I have stayed at this Marriott in Waikiki with the family (4 of us). This included two kids around 10 & 12. It was an award stay, and I was not charged any extra amount.

I have called the Plat line several times, and asked them to call a property and see if they will accept two adults/two kids. Oftentimes the extra charge is only for 3rd and 4th adults. See if the plat line can get it straightened out for you.