Everyone worries about this. First of all, to be honest if you are planning to go on and do A-levels and a degree at university your GCSEs won't be the most important qualification when you come to decide a career. You could really do any combination of GCSEs and still be able to do either of the two subjects you mentioned as your career. If I were you I wouldn't worry too much now about what particular subjects you want to take because there aren't any GCSE subjects that are directly related to animal work or working with children anyway. I guess as long as your doing your core sciences, including biology then you should just choose the subjects your best at and you enjoy most. Its once you've completed GCSEs after year 11 that you should concentrate on studying towards your chosen career. Basically, if you get good enough grades in your GCSEs, no matter what the subjects, you can do anything you want after school.