Hi there, Julie!
The Disney Dining Plan is a great way to pre-pay for your meals prior to your Walt Disney World vacation! It is very age-specific, however. You cannot get the adult plan for a 9-year old just like you cannot get the child's plan for a 10-year old.
I do have some advice for you that you might find helpful though. My 12-year old did not like the options on the adult menus when she was 10 and at most restaurants, we were able to order a larger sized portion of the items on the kids' menu. There was an additional charge to do this, but it wasn't much and worked out really well. What you could do in your case is ask your server if your 9-year old could order a smaller portion of something on the adult menu. Of course, I cannot guarantee that you will be able to do this at every dining establishment, but I have found that the Cast Members are very accommodating and try to do whatever they are able to in order to make it all work out.
I sure hope this helps, and don't forget,
Advanced Dining Reservations can be made up to 180-days prior to your vacation either online or by calling 407.WDW.DINE [939.3463].
Wishing you Pixie Dusted vacation planning!
~Jackie