Must I discourage my children from taking certain things which may not be good to them?

No, never, that is the very bad tactic; moreover the children’s curiosity increases.
You know from your childhood, the most interesting things are the one which is not allowed or prohibited. You allow them to taste everything and explain them why it is good or it is not and tell them also they could eat it rarely. In the long run they slowly omit these unwanted things and explain their friends also why they should not do it. It is better to allow the children to do the mistake, then they have enough time to correct, but if they do the mistakes when they are adults, you may not have the possibility to correct this. This is applicable to all fields.

But the parents say they have no time and their knowledge is limited, what to do then?

This argument is known throughout the world. Both mother and father must spare time for the child; you can talk to the kindergarten, school and friends. Moreover, you can request the grandparents to take time and talk to children about this topic. I know the children may not have time as they are fully occupied with their home works and other activities. Spending 30 minutes twice in a week will help you a lot to learn about your food. Where there is a will there is always a way.