I heard again for the millionth time someone exulting over the cute thing that some child had said that by claiming that the wonderful thing about children is that they are so honest. They're not honest. Children actually fib quite a bit. What they are is candid, usually because they haven't picked up enough of the rules of social interaction to know that "there are things we just don't say, dear." Sometimes candor is welcome, pointing out the obvious that adults are evading. More often, it is hurtful and awkward and involves having to apologise for them to a person who tries to laugh and say "That's all right. It's no secret that I'm fat/old/black/foreign/disfigured/tatooed."
Similarly, "Can I be honest with you?" means may I be candid. Honesty is the best policy, but candor is mixed.