If you are all adults in the same room yes everything will be pooled together. You can can use your credits however you choose until midnight on your check-out day. There is spot on the app where you can see what you have left. I have not tried, but would assume you could mobile order an alcholic beverage using your under 21 adults credit as long as they are not drinking it. I believe I have read others have done that
If you had kids their quick service and table service credits would have been separated.