As the student matures, you can offer a more structured program. Older children have developed their motor skills to such a degree that they can learn many more of the fundamentals and effectively apply them to their artwork. Evaluating children's artwork takes confidence and practice. Another way is to hold the drawings up to a mirror to see a reverse image of the picture and point out many of the mistakes. Finally, when evaluating, balance praise with constructive criticism.
Every student becomes frustrated at one time or another.
Refocus their attention on having a good attitude and doing the best they possible can. If they are having difficulty, assist them with the art lessons. You can also go on to another assignment and return to art at another time. Mood has a lot to do with art. Don't insist on art time.