I've only been to Krabi, and only to Phulay Bay in terms of Thailand, but I can certainly highly recommend it. Service was unparalleled, food was excellent, activities (cooking class, visit to local markets and shops / town, fitness facilities, spa, pool, beach) were all great. I'd be careful about time of year you go to ensure you're there when the weather is best. I was there in the low season, and it was cheaper, but the weather was less than ideal. It was INCREDIBLY quiet there. I was literally the only person at dinner in the restaurants most of the night even though the resort was near half full. There were a couple couples with children and they were fine. I think the place is generally so quiet and what not that nobody would notice a few kids running around.

Anyhow, I'm glad to answer questions, just let me know. The villas were fantastic. I was in a Royal something or other. The only consideration I'd have in that villa was safety of the private plunge pool depending upon the child's age, and the bed (which is AMAZING and incredibly massive) is still only a single mattress, there didn't look to be a natural place for another bed in the room. But that bed is big enough for about 6 people, so I think you should be just fine.

Anyhow, let me know if you have any questions...