I'm in my first year studying child nursing (: and I love it; it's amazing.
GCSES - you will need a minimum of 5 GCSEs with C and above. The 5 have to include English Literature, English Language, Maths and Science. I recommend you take GCSE heath and social care as one of your GCSE options (counts as 2 GCSEs) as it will help you out a lot, it will be relevant and you will get a hospital placement so you will be able to shadow a children's nurse and see what they do. You also want to take a subject that's academic and will make admission tutors that you are not a one trick pony, so something like history or geography will be good. I did history and loved it. As long as you pass all your GCSEs with C's and above you'll be fine.
A-Levels: You need a-levels or the equivalent. The best a-levels to study are: health & social care, maths, human biology, chemistry, psychology, sociology and english literature & language. Take 3 and aim for B's and C's. Typical offers are BCC, or BBC. You can also do BTECs which the majority of universities accept even the top nursing schools. The best BTECs to do are BTEC science, BTEC Child care and BTEC Health & social care. Typical offers will be DM (equivalent to AB in a-levels) or DMM (equivalent to ABB in a-levels). I did a BTEC and a few a-levels and got into all my choices: kings college, nottingham, southampton, st george's and east anglia. BTECs are good as they will prepare you for the course work load and you will also get 2 weeks of work experience so you will be able to apply to a hospital if you want.
It would be wise to try and do some work experience in a hospital or a care home. As you want to work with children you could do some work experience in a nursery, school or a mother & toddlers group so you know a lot. Work experience will help and set you apart from the crowd.
Good luck HUGS XOXO
student child nurse