When working with parents of young children, a simple Goal Sheet can be a game changer. This printout gets to the point without any frills. First, write down the goal for the week. This could be a chore ("clean my room every day"), or avoiding an unwanted behavior ("I will practice my anger management skills when I'm upset").
In addition to writing out a simple goal, take some time in session to define exactly what it means. Be careful not to use goals that are too broad, such as "I will be good all day". What constitutes "good" is probably very different to a parent and child, and disagreements after the fact can be a real problem.
It will be the parent's responsibility to record their child's success meeting their goal each night, and to provide an appropriate reward after an agreed upon number of completions. It's a good habit for the parent to discuss why their child did or did not reach their goal each day.