If, for example, an adult tells a child not to eat ice cream because it's bad for them, it would not likely stop a child from wanting to eat it. The ice cream might be a bright color (think bubble gum flavor blue ice cream) and be fun because it comes on a cone. Plus, they like the way it tastes. If it looks good, causes the child to have a good time, and it tastes good... how could it be bad? As adults we understand that adult probably really meant that the ice cream was unhealthy, not bad.
As children grow and begin to face more difficult challenges than when to eat ice cream, the need for precise language increases all the more. If this does not happen, children will not be able to reconcile why the things they've been told are bad seem so good. However, if they have been taught a certain action is sinful - as opposed to bad - the question of how it feels is secondary to God's command.
It also allows parents to have much more honest conversations with their kids: Yes, sex is enjoyable, but God wants you to reserve that for only your spouse. Yes, you will make a few more dollars working every day of the week, but God wants you to rest your body and spend a day close to Him. Yes, lying is a sin even if it doesn't hurt anyone.
It is our responsibility as adults to make sure our children understand God's law. We were instructed to us do so. In Deuteronomy 11:18-21 we are told to "Remember [God's] words with your whole being. Write them down and tie them to your hands as a sign; tie them on your foreheads to remind you. Teach them well to your children, talking about them when you sit at home and walk along the road, when you lie down and when you get up. Write them on your doors and gates so that both you and your children will live a long time in the land the Lord promised your ancestors, as long as the skies are above the earth." (NCV, emphasis added)
What about you: Is it hard for you to talk about sin with your children? Other adults?