In today's era of trophies for the losing team, it's important for kids to learn how and when to push themselves to do better. Shakespeare didn't pen "Romeo and Juliet" the first time he picked up an inked quill -- it takes time to develop skills and talents. Rewarding your child for mediocre achievements gives the impression that OK is good enough.
Now, we're not saying that you should constantly find fault in your kid's work -- we're just pointing out that if your child is practicing writing sentences but neglects to include verbs, you might want to show him how much those action words can improve his prose. Adjectives and adverbs, however, can wait for another day.