I am also in the Something Must Be Done, But Probably Not This camp, for all of these reasons:
There's no action plan yet in the campaign proposal. You say "a Mumsnet campaign". Does this mean "the community" will have the chance to sign a petition, send a form letter to a retailer and a link to their Facebook friends? Given the huge community you have already have here, so much more can be done.
In any MN campaign it would help to have practical actions that supporters can sign up to. Quick simple things we can do, that in doing, also encourage others to get involved. There were some fun suggestions on the OOA bad advertising thread, like meeting up to dump dirty nappies in the ad agency's offices. Flash mobs of sarcastic mums can figure out how to stay on the right side of the law and also have fun.
But right now the campaign message is negative - "stop doing X". More positive campaign would help - "encourage doing Y" - . Positive campaign will support morale of people involved in it; a negative one will attract flames and put people off.
A positive campaign message would help focus on the real goals. With a negative message you risk negative coverage, attracting Mary Whitehouse type sympathisers, encourage people to yell at each other on the internet even more, just what the advertisers would like. And if you can't think up practical actions for supporters of your campaign, are you doing any good, or just helping to get people worked up?
Retailers need a business benefit to change their practises. It's easier to sell a carrot than a stick. Persuade just one big chain to present clothes and toys in one space with no gendered boundaries, more neutral colours, etc, and that would draw a lot of mums there and get the other stores' marketing people worried. Fairtrade, organic, renewable, etc, is all very nice, but if it did not have a marketing and money benefit for stores, they would not stock that stuff.
In a couple of places the text at www.mumsnet.com/campaigns/let-girls-be-girls talks about "children" where it really means "girls". There is already an overfocus on girls. They are hassled over appearance enough, are the object of others' decisions enough. Campaigns like this are for everyone, for future society's sanity.
A couple of people pointed out Pink Stinks, trying to do very similar things. MN staff could work to promote their campaign and ask MN mums to support it. That would also show that it is about the cause, not the coverage, for MN organisers.
Okay, done for now.