My child talks to strangers or goes off with them.

Teaching "all strangers are dangerous" makes it hard to know who is safe and can actually cause confusion. Decide concretely on people it's okay to go with (a set adult, like family or teachers) and people to rely on when in trouble (a store clerk, the police), and make it a promise that "before going with any adult, always check with an okay-to-go-with person." Also teach that an adult asking a child for help, or telling them to keep a secret, is a strange sign. Practice, situation by situation, refusing, leaving the spot, and telling the set adult.