My family of four (including two boys age 3.5 and 5.5) had a great time on the O! My older son was old enough for Club HAL (by himself) and had a blast everyday.
Our head steward in the Vista Dining Room and his supporting cast were very attentive and dealt with our son's food allergies and his very restrictive diet in a very personal, customized, off-the-menu kind of way. To be sure, we had to fill out an SRI form more than 60 days in advance of our cruise, but once they had all of his food allergies documented, HAL pre-purchased food for him for the entire cruise (food not normally available on the ship in large quantities).
Regarding Club HAL, it is said that all HAL ships will be re-configured to handle children as young as 3 years of age (to be left without a parent/guardian) by the year 2006. As for now on the Oosterdam, our 3-year-old was able to attend all Club HAL events but only with one of us present.