When we instruct our children, we offer them a voice of experience.
I've always believed it's best to be honest with children (in an age-appropriate way, or course) about the challenges we have encountered in the adult world. They may not need the whole truth all at once, but they will face many of the same challenges, and perhaps they can learn from our mistakes.
Few of us enjoy being "preached at," but advice is much easier to take when offered humbly, as one who has walked the same road and learned where the rough places are. By instructing our children about the realities they will face, we literally build in wisdom.