Many public and private schools will include test prep in their regular English and math courses. Others will provide separate classes for SAT prep, like you are doing. It's completely fine to give credit for test preparation - either by using it as a component of another class, or by given credit as a stand alone class.
You can call it critical thinking, or study skills, college study skills, college readiness skills, or something like that. I prefer "Study Skills" unless there is a critical thinking curriculum or discussion in your study - but that's just my opinion.
For credit value, estimate the number of hours spent. If the student works 2-3 hours per week for most of the school year, then call it a credit.
Your best strategy for keeping all those balls in the air is preparation. The HomeScholar Gold Care Club will give you the comprehensive help you need to homeschool high school.