Some Koh Lanta hotels and resorts let children go free if they are under 2 years old and sleep in their parents bed. If the child is over 2 years old, an extra bed or a baby cot will be required and at some hotels you may have to pay extra for this. It’s still much cheaper than having to hire a second room for your kids.
For example, Rawi Warin Resort has this “children go free” policy.
Children and extra beds:
Children under 2 stay for free in existing bedding. Note, cot charges may apply.
Children aged 2 or over must use an extra bed. An extra bed is available for this room.
Children policy is listed on each hotel page in Koh Lanta Hotels section.