I can't recommend the Safe Side series http://www.thesafeside.com/ enough! It is wonderful! BUT At the ages your children are- they aren't going to get it. It just doesn't really register at 2 & 3, unless they have an innate fear of strangers. My two girls are extremely social & gregarious. They love to talk to & engage any & all. At 5 & 8, I have to remind them constantly to be vigilant- not to trust anyone I don't tell them it's OK to trust!
Danger is everywhere, but we cannot & must not live in fear. The good people are the majority. Bad seeds have always been with us- you just didn't know it when you were a kid b/c there was no 24 hour news!
D.