You can use your 100 credits to pay for any of the exercises/tests in the online testing section COAsT.
There is lots of support information and FAQ in the tab at the bottom of the home page. I suggest that you have a look at this and the helpful videos.
It is a good idea to use your credits, or some of them, to pay for your daughter to have a go at some of the previous years’ mock tests as this will help her prepare for the test you have already booked. When you look in COAsT you will see that each of the tests/exercises will show you how many credits are needed.