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.
A fun activity is to make a marine excursion in which fruit is offered to the fish inside the water, they love it and will surround you looking for their bite.
Khao Lak, Andaman Sea
Phuket, Andaman Sea
It is the largest island in the Andaman Sea and offers excellent beaches for families.
Many beaches on the west side like Karon or Kata have a fun wave on the shore, big enough to have fun but not too big to become dangerous. Take into account that it changes during the monsoon season, with a less friendly sea.
Post about Phuket here (enjoythailand.fun)