Hi Colleen and welcome to the Disney Parks Moms Panel.
Unfortunately, it is not possible to use a Child Table-Service Disney Dining Plan meal entitlement towards the cost of an Adult meal and for you to pay the difference.
As stated in the
2016 Disney Dining Plan brochure, Guests ages 3-9 must order from a children’s menu where available.
If your granddaughter wanted to order a meal from the Adult menu, you would need to pay out of pocket for this.
The good news is that there are lots of different options available on the children's menu for your granddaughter to choose from. As a mum of a very picky eater, I don't think that you will have any issues with finding her something that she would like to eat from the children's menu at any Table-Service restaurant.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma