To put a little perspective on this--he was only 7 years old, so happened a long time ago. And the fact that he's even owning up to the wee tale is pretty sweet--it's not like he'd ever have to admit it.
He told Top Of The Pops magazine (via PressParty) recently: "When I was seven, a girl I really liked tickled me and I wet myself. Then she laughed, 'cause she was a lot older, but I cried and it was terrible. I actually really fancied her and I'd ruined everything."
Scarred for life? She may have the ultimate "I knew him when..." story about Liam, but he said all's good, explaining: "I saw her the other day, 'cause she came to our concert. So it was one of my redeeming moments: 'You just came to my concert so now you can let me off wetting myself.'"