Wanna own a piece of Cristiano Ronaldo history AND potentially save a child's life???

Good news ... the armband thrown by the soccer legend in a fit of rage last weekend is hitting the auction block, and the cash it nets will go to helping a 6-month-old who's battling a serious illness.

You'll recall ... Ronaldo threw a temper tantrum last Saturday after his apparent game-winning goal in Portugal's World Cup qualifying match with Serbia was disallowed.