One of the reasons babies have such soft skin is that they haven't been exposed to the harmful effects of UV rays. Wear a hat and cover up with long sleeves and long pants/trousrs/skirts if you are going to be out in the searing sun for any length of time.

How would you rephrase that briefly using Vietnamese?
Bôi kem chống nắng mỗi khi bạn ra ngoài!