Hi kimcheese (interesting forum name
I can say that Patong Beach is family friendly and your son will have a good time there. Kata Noi is another nice beach to take the family,about 300baht by a/c taxi or tuk tuk.
Perhaps a speedboat trip to Phi Phi islands for the day.Superb scenery and well worth a trip.
Phuket Zoo would be an interesting place for a 3yr old and makes a nice half day out. Possibly the Cobra show at Wat Chalong may interest him. Its hard to know what 3yr olds like
