If they don't apply sunscreen on their skin, the sun will burn them easily
好似好中式英文!!
1. 塗,敷;將...鋪在表面[(+to)]
The nurse applied the ointment to the wound.
護士把藥膏敷到傷口上。
If they don't applied the sunscreen on their skin, the sun will burn them easily
Or
If they don't use the sunscreen on their skin, the sun will burn them easily.