Credit works on luggage and seats, not sure on the kids. Just make sure you are adding them to your purchase, they may not allow you to use credit after a confirmation code is issued.

Discount memberships are definitely not going to work with credit. Even in good times I tried and they refused, saying its "a different department". They probably book that revenue to a marketing department or something non-aviation to avoid taxes and regulations. Back in the day I had a similar problem with Spirit and getting their $9 fare club with some credits I had that were going to waste.