This advertissement is for adults in my opinion, and more specifically for to move the adult donnors. Not more, not less. Is very ineffective, and probably very expensive also.
You are not reaching the childrens living in the country, nor the childrens locked at home, or at the boarding schools, neither the babies/very small childrens that can not read still, nor the alienated that don't even notice that they are abused or that think that they "merit to be slapped" because they are... "too bad, too silly, too stupid"... people are very good lying to ourselves
And even if you reach a desired target they probably don't call you, a perfect stranger.
An abused child is frequently intensively surveilled, most of the time they are keep in a controlled environment, at home or in a small group of related people. Rarely could access freely to a public telephone and I bet that this boy or girl will avoid carefully (fear, shame...) to look directly to the advertissement shown for a few weeks in a random street of a big city.
> You said school. Can you name a few more?
School is basic, as health services, you can also add libraries, and TV.
In Spain an abused child goes to an hospital sooner or later, and some injuries are very dificult to hide or fake to a professional. A physician can take a discrete look under the shirt of a child and he/she knows what to search for. Later they can send a silent alert to the appropriate channels if this is suspicious or simply call the police if there is some gross evidence.
If you want to tell a telephone number to the children, simply assure that this number is a the school, in a poster at the hall. Pretty simple, safe and cheap also.
If you want to be really twisted, put this number in some selected books in the school's library with an inked rubber stamp.
Ask the teachers to spoke about it in a delicate and sensitive way. They can provide a better, more focused and controlled message in a safe environment. A poster in a solitary street with this aggressor can' have the same benefits and could even put him/her at risk. The adult could even beat him, menace or manipulate him distorting the message in unpredictable ways
If you want to reach a target not in education you could use the TV channels for childrens. When we are trying to sell a toy, a film or a burguer to a child there is not a safe place where the parents can hide. You can also introduce this theme in programs like sesame street, in a subtile way and with an apropriately, positive and carefully expressed message.
You said school. Can you name a few more?