WWE Superstar John Cena -- GRILLED By Adorable Children

WWE superstar John Cena was TRIPLE-TEAMED by some curious kids with some hard-hitting questions recently ... who made the wrestler divulge one of his BIGGEST SECRETS EVER!!!

It all went down at an event for the Make-A-Wish foundation, which was captured by a crew for his upcoming documentary "Once In a Lifetime: Rock vs. John Cena" ... set to air Monday on USA.

Cena promised to answer any question they asked ... and of course the kids dropped the BIG ONE: "What's your favorite ice cream flavor?"