Weak-sauce is best used as an adjective to decribe how pitifully someone has failed when they were actually trying their very best.
When motivating your children:
"Really son, is that the best you can do? What is that? A TURD!!! You are supposed to be drawing a picture of your family for this homework assignment! Do you hate us? Would you prefer living on the street? That finger painting is so WEAK-SAUCE!!! No, you are weak-sauce!