Fiji! We recently stayed at the Radisson Blu with a 9mo and 3.5yo. We went with friends and all the kids went to kids club twice (3hr afternoon session) - we hired a nanny for the kids that were under 4 but they all stayed together and did the kids club activities together.
The rooms are a good size, get the 1br apartment - then you can put the kids to bed and close that area off. The pools are great, lots of shade sails and there's a waterslide! Plenty of activities and other resorts to visit for different restaurants etc.