Buffets

[chrim]

These aren't buffets, so perhaps they are way off the mark, but are pirate themed:

morning boat trips for kids only:
WDW Pirate Adventure Cruises

evening boat trips for adults and kids:
WDW Pirates & Pals Fireworks Voyage

On the other hand, while not a buffet, Whispering Canyon Cafe does have an "all-you-care-to-eat" dinner platter option that might work. No pirates, but perhaps cowboy or old west themed attire could work for your grandchildren (if they would want to dress up).

I haven't tried the Garden Grill, but it might work if you want characters (and since you already have plans for Chef Mickey's).

The Tusker House in AK park might be good if you want more non-standard food options (and yes, I realize this is also known as the Donald's Safari Breakfast).

[DANAM]

evening boat trips for adults and kids:
WDW Pirates & Pals Fireworks Voyage

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My brother and sister in law did this with my 4 year old nephew in October and she still talks about how wonderful it was. It was chilly and they provided blankets for everyone. Extra attention with characters. They said it was well worth the money.

I was going to suggest Hollywood and Vine. Jake, who is a pirate, is present. Not sure of the age ranges your boys are. My son was 9 last trip and was kind of over it but when he was younger he loved this place. We always enjoy the lunch here. Breakfast was okay and dinner doesn't have characters. I have booked the Fantasmic package at lunch which to me is a double bonus. Seating forFantasmic and Characters