Spent a couple of split nights there last week with my wife and three kids. Used the Diamond cert for breakfast in Red Marlin for only four of us on first stop (youngest wasn't feeling well and didn't eat). On second stop we had to have our breakfast in the Swim lounge as it was Easter Sunday and the Red Marlin was fully booked until the afternoon. They comped all 5 of us the breakfast buffet they were serving in Swim that morning- probably a one-time deal due to the crowds there for the weekend? Even if they only allow you 4 breakfasts in Red Marlin, if you order off the menu 2 breakfasts should be plenty for the 3 kids to share, providing you can convince them to peacefully share.

Mile Hound and BlissWorld (farther above) are both spot on- the Hyatt Mission Bay is an excellent property! Truly a gem, and a great place for kids. If you have kids and you're considering a Hyatt in San Diego, then this is the place for you. The kids will love the pool slides and roasting smores over the firepits. You'll love the cheerful and accommodating staff. I miss it already.

p.s. they gladly accepted the costco certs.