I think a new thread isn't too much. Probably. 
I'm leaning towards thinking it's a good idea anyway, as if it's a launch, then it's easier if people see a fresh thread, rather than this one with nearly 500 messages to trawl through!
Don't forget the two survey threads will disappear in a few weeks / once we have the data.
Also, this is the campaign thread, so we're all in agreement here.
But once we launch the idea, people will want to debate the toss (this is mumsnet after all!) This thread is working brilliantly IMO. It'd be a shame to have this thread full of people arguing about whether it's a good idea or not!
We could use the other thread to announce stuff (kind of like the public face of the campign on MN) and for getting involved with debates with critics about whether marketing affects children. And save this thread for planning.
And anyway, if we're promoting this, the more threads the better, surely?!
What do you think?
Btw, can I ask which talk section it'll be launched it in? Reckon we need one with good traffic (but not AIBU of course, that would be just asking for trouble!) Is chat the best for traffic? Or somewhere else?