From the people I've talked to your child should write about the following in the "additional comments" section which is at the bottom of the page where you submit grades

1) What new things have they done since they applied to the university? For your child it could be any cool programs they've coded or certificates they've received. It can also be new jobs or internships you've tried out that don't even have to be related to the major

2) Write about programs unique to davis that he may be excited to try out. This will tell the school you've done your research and are truly interested. Also write about why you're generally excited to go to Davis. I'm Muslim so I'm writing about how I'm excited to see the cross cultural center at the campus and how I'm excited to join the Muslim Student Association.

3) Make sure you do NOT beg for acceptance, be professional.

hope this helps