Refer to the assignment book for weekly practice guidance on focus elements. Work on the most challenging measures before playing the entire piece.
For any beginner, but especially younger children, 15-20 minutes is long enough. Two short sessions are better than a long one.
Verbalizing finger numbers, note names, note values and other elements improves focus and reinforces learning.
Be supportive, but not a second teacher. Just sitting by the student during a practice session and appreciating their playing is all they need. Having multiple teachers leads to confusion.
Positive reinforcement is optimal in order to learn and develop good habits. (Don't punish mistakes -- that's just negative reinforcement.) Maybe have a cookie! If a daily practice session isn't going well, take a break.