This is a great idea. I was a Code-in participant in the past and enjoyed it (even though there were basically no web-related tasks back then – hopefully MediaWiki can fill this niche ;) ).
Skills and eagerness of young wannabe-programmers are often underestimated, Code-in is a lovely project to fix this misconception.
I am volunteering to mentor and take part in preparing task list, to the extent that my free time allows. What is the timeline for this?