Hi– this is such a challenging issue — for any parent to talk to their child about! It is always such a delicate balancing act– you don’t want to scare them but you also want to keep them safe.
This article is really helpful (or at least I found it very helpful): https://blog.stageslearning.com/blog/teaching-autism-stranger-safety
Let me know if this helps?
Best of luck to you! Leslie