Hi I would suggest to go to Koh Lanta, it is really well setup for family with kids. You can choose to stay high end but there's also a great selection of more medium and basic accommodation. The Island has banned "adult" bars, and there's only a few clubs, so you will not get disturbed by noise. It is a bit a of a trek to get to, but the hotel where we were staying had car which picked us up from Krabi airport (for a small fee) and took us to the ferry and then the hotel. Enjoy!