I think it's a cool idea. The stickers look like they'd come off pretty easily, and it wouldn't be beneficial to UNICEF if they just got all nasty and gummed up. Plus they're huge, you'd be pretty oblivious not to notice that right away.
And if you actually read the ad, it said that the stickers were placed in front of donation booths, and that their donations rate increased... so, not really a waste of money... (and even if it didn't increase their donations, at least they're informing otherwise oblivious people about the tragedies that happen in many countries...)