Is there a time when we should be honest with our children and thier talents?

We went to a festival and there was a girl singing on a stage and the poor thing god bless her wasn't very good. I give her all the points in the world for bravery and such. But my question is "Is there ever a point where you have to tell your children that something isn't the best for them." Like could you tell your child she/he will never be a rock star? Isn't it cruel to tell them but wrong to let them think that they have actual talent. Should we let them find out on their own? Look at all the talent shows with deluded adults and teens making fools of themselves.