Melissa Rivers -- XMAS Wish ... I Want My Son to Laugh Again

This will make you cry.

Melissa Rivers had a very simple Christmas wish, which encapsulates the sadness and loss she experienced this year over what seems like the preventable death of her mom, Joan Rivers.

Melissa was leaving Craig's restaurant in West Hollywood when a photog asked the simple question ... she wants 13-year-old Cooper to laugh again.

It was clear from the funeral ... Cooper is taking his grandma's death very hard.