The table service credits are only split adult and child on a card. There is no using 'her' credit - they are pooled.
So 2 adults and a 10 yr old checking in for 5 nights get a card with 15 adult TS credits on it as they are all disney adults.
How they spend them is up to them eg one person could use them all, they could each have 5 TS meals etc.
What the op is going to do is use 2 credits a meal then pay out of pocket for her 10 yr old. A kids meal is $8 or so. A credit could be worth $40+ at some places.
If they do this they will be able to have some more TS meals or eat at places needing 2 TS credits a person - signature dining.
The problem on the boards is usually people complaining the kids food is not enough and paying to upgrade 8/9 year olds to disney adults.
But if you have a 10/11 year old light eater this seems a good plan.
But it only works if your child is age 10 plus.
TS credits are split adult and child on
DDP so you can only get a childs meal with their credit eg 2 adults and an 8 yr old for 5 nights get 10 adult TS and 5 child TS loaded on their card.