I have dealt with this situation.
For park entry purposes, once a child is 9, until they become 18, they are considered a Junior. They still need an adult park pass, as they are usually tall enough to enjoy all the attractions offered by Disney.
For the resorts - Jrs are allowed to stay free in a room with their parents as long as they are within the limit for number of people in a room. Once they are 18, there is a charge for more than 2 adults per room.
For Dining - the age cut off is different. Kids under 12, i.e., 11 and under, are considered children. They will pay the reduced price for meals at buffets, packages etc., so your soon to be 10 year old will have no problem.
My daughter, now 13, went when she was 11. As she is a very big girl - currently 5'9", size 11 shoes, I got a lot of funny looks from the people at the podium when I checked in with 2 children, 1 junior, and 2 adults. They assumed my older, but shorter, daughter was the child. I never was asked for any ID for her - but I did carry her birth certificate just in case! Even though she looks much older than her age - she is an extremely fussy eater. She will not eat much of anything on an adult menu - chicken nuggets, macaroni and cheese, or hot dogs are her idea of gourmet food.
We went to the Candlelight Processional last year when she was 12. When I called to purchase the tickets, I asked if she could order off of the child's menu, since there was nothing she would eat off the adults menu. (We were eating at Le Cellier). We did eat at several buffets during the trip, and at these, I happily paid the adult price, since she was according to the rules. The Dining CM did allow me to purchase 3 adult and 2 child packages, as we were dining at a restaurant with a separate menu for each. Why should Disney care if they are getting paid for the menu item ordered? Again, there was no problem at the podium, they just asked who would be receiving the child's menus.
One interesting aside, even when she was legitimately under 12, and getting kids meals. At Epcot, when you eat at the sit down restaurants, where a drink is included in the kid's meal, you get a neat souvenir cup - just the right size (10-12 oz), with a cover and a reusuable straw that seals the cup. We have quite a collection from our trips from my youngest - but Jennie, although ordering the same meals - never got one!!! They always brought her a drink in a glass. Just because she's big, she's supposed to be coordinated??