Ko Olina has nice beaches for family. It has four lagoons, so the waves aren't big and scary for little ones. The beach at Turtle Bay is also VERY family-friendly and again, a lagoon so the waves are minimal. Turtle Bay is the farthest from the city - it's on the north shore and will probably take you about an hour from the airport if there's no traffic. Ko Olina would be about 45-50 minutes from the airport without traffic. Ko Olina is closer to a lot of cities and Turtle Bay is more in the countryside.
I've never stayed at any of the Ko Olina hotels, but Turtle Bay is an awesome resort. They also have a swimming pool. Both resorts are REALLY pricey, so if you're concerned about saving money, Waikiki might be a better choice, although I have no idea what the beaches there are like.