In our opinion, all beaches are excellent destinations for families.
We would only discard Koh Tao if you are not going to focus on snorkeling or bottle diving. It is far from the continental coast and the beaches on the main island are not the best. The access to the beautiful Nang Yuan is by hiring a long-tail boat, so not free.
If you opt for an island, keep in mind that if the sea were rough the ferry trip would be very uncomfortable, especially in monsoon season.
If you may reach it from two ports, choose the nearest one. For example, to go to Koh Samui and Koh Phangan, better do it from Surat Thani and not from Chumphon. Although in this case, you can also go by plane.
The destinations that we are going to suggest can be considered as very suitable for families, but why?
» They have wide beaches, very quiet.
» They are not the most spectacular, so there are no crowds and the offer of accommodation is wider and affordable.
» Nevertheless, There are nearby paradise islands that can be visited on day trips or include them in the trip itinerary, combining quiet beaches with authentic marine treasures.