Can you please tell me which area in Hainan is good for young families in May?

You may go to Sanya. There are many interesting places, like Tianya Haijiao, Asian Dragon Bay, Dadonghai, and Wuzhizhou Island. For accommodation, you can consider Grand Metropark Resort at Yalong Bay National Tourism Holiday Resort. The hotel provides clean environment and quality service. Have a try!
By the way, it will be quite hot in May. Remember to pack summer clothes, as well sunglasses and sunscreen.
Have fun!

Thank you