You're absolutely, right, JAS84.
Having just done a little research, it would appear that in 2003, it was broadcast on ITV, and was then broadcast on ITV2 in 2004 and 2005.
Having never watched the Junior edition, I was not aware of this! Thanks for the information.
That's right, they took it quite seriously in 2003 with a qualifying show and then the contest live on ITV on a Saturday night, shifting Pop Idol to teatime to make way for it. However it was hugely unsuccessful, not helped by commentary coming from Mark Durden-Smith and Tara Palmer-Tomkinson (as ITV had just started their policy that if you were at all popular on I'm A Celebrity, they'd use you on at least half a dozen shows after it), who were pretty hopeless and completely failed to work out who'd won, still speculating if the UK could win despite it already being mathematically certain.
ITV were supposed to host it the following year but they pulled out of that because it had rated badly, and they showed the next two years on ITV2 to wind down the contract. I think they might have repeated the 2004 one on ITV on a Sunday afternoon, but not the next one.