CS credits are pooled in that there is no distinction between adult and child credits. Your TS credits are not assigned to any one person, so as long as you are looking to pay for another adult (or a kid, though that would be a poor use of the credit) using your TS credit at the same meal, that is totally fine. You just can't take a child TS credit and use it for an adult meal.
On the Deluxe plan, there is no distinction between adult and child TS credits.
Yes, you could (for example) eat at a signature and use 4 credits for your mom and sister to eat full meals while buying yourself an appetizer out of pocket. It's a good way to use credits wisely.
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