This is a fairly recent addition, and I have to confess that I've never seen it happen or heard of anyone falling victim, but I've read about it on Barcelona forums and thought it might be worth adding.
On the occasions that the scam has unfolded, it's been a Sunday morning, fairly early (so that's like 10am for those of you yet to get to grips with Barcelona lifestyle) and involves children and a supposed questionnaire.
It seems that those targeted were usually couples, although I suppose the scam could work for a small group, too. This scam relies on Innocence. The children were alleged to be around 12-13 years old, which is smart enough to scam but also innocent enough to be believable. Here's how the scam unfolds...
Lazy Sunday morning, and a couple deciding what to do for the day are mobbed at the bottom of Las Ramblas/Christopher Columbus monument/streets around Carrer Ample but a group of young teenagers, asking them to fill in a school questionnaire complete with clipboard and pen. They speak a foreign language (some reported that they didn't seem Spanish), and are very in-your-face.
It's all very confusing for the holiday couple who try and explain that they don't understand, while the circling youths seem to insist and even look offended that they willingly want to damage this terms grades. The couple lose sigh of each other, and then the children run off, in the easy access of the Gothic Quarter nooks and crannies. Tourist couple dust themselves down, annoyed, and realise that their backpack is open/wallet has gone, etc. Clever little pickpockets.
Now this is not your usual distraction technique, partly because it's the LAST thing you expect from young kids, and I think this is all part of the success of the technique. As a consequence it's a little damning to offer advice "to stay away from little kids in the Gothic Quarter" as this will be virtually impossible and also may ruin your holidays and turn you into a paranoid maniac! I suppose the tell-tale sign to sit up and take notice would be the clip boards, and if you see them, to try and shoo the kids away without looking like 100% unfriendly tourists by the onlookers!
For planning your trip against any possible mishaps, see this detailed Barcelona Guide and check out my other recent posts on Barcelona Scams:
The Reverse Parking Scam
3 Cups Scam
The Map Readers Scam
The Football Dance Scam
The Flat Tyre Scam
The Ketchup Squirt Scam
The Dancing Dollies Scam