Three weeks ago, on a Thursday noon, my wife and I were walking near the Puerta de Alcala when two girls with clipboards blocked our way and asked us to sign a petition for a supposed good cause. I shouted, "No!" and we walked away. The girls were probably pickpockets and/or identity thieves, working for a mafia. They were Eastern Europeans and under the age of 14. In Spain no youth under 14 can serve a jail term for theft.
Spanish youth under the age of 16 will be in school because this is mandatory by law. They will not be on the streets. Honest Spanish people tell their children not to talk to strangers. People from legitimate charities are in their twenties and will wear a vest showing they belong to a charity. They may ask for donations, but never will ask you to sign a petition. The petition will ask for your DNI, the national identity number, which foreigners will not have. The gullible Spanish who sign it will have given away their name and DNI, and the identity thieves may wreck havoc with their lives.
These people know you are a foreigner by the way you look, the way you dress, and the way you walk. You are their target because they assume you are innocent to their tricks. If you stop, while one talks to you, the other may try to pick your pocket or purse.
My advice to you is to say no and walk away. Remember that most honest Spanish people will not talk to you unless you initiate the talk.