Spider diagram on trying to choose what topic:
I googled the four topics:
say no to stangers
The one that stood out for me was recycling as can inform with information and can also add fun bits like creating objects out of materials you usually chuck away.
I am not going to see what campaigns are already around and how they aim themselves at children.
http://www.recyclenow.com/schools/primary_school_resources/busta/index.html (probably the most heard of campaign)