I don't think its necessarily a case of choosing the field with the most scope... as each field has a huge scope of employment prospects within them! Think about why you're doing your Nursing access course, where you might want to be once you're qualified, consider any of your personal interests etc to help make a decision!
Here's a bit of info on each field/where it can take you:
Children's NursingLets start with this field since you've mentioned an interest in it - which is already a fairly good indicator! Children's Nurses work with people under the age of 18... so you could see anyone, from newborn babies to teenagers. Children's Nursing requires great communication skills (as children usually don't communicate in the same way as adults do) and it's often a dynamic environment as children's health can change quite rapidly. Children's Nurses can work a vast number of areas, including (but not limited to) medical wards, surgical wards, Paediatric A&E, Children's Hospices, Health Visiting, School Nursing, as well as other Paediatric specialist areas like Burns & Plastics, Cancer Care, Neonatal Intensive Care, Child Protection, Cardiology, Respiratory, Diabetes... and so on & so forth
Mental Health NursingMental Health Nurses can work with both children & adults, and they support & promote recovery or better management of an individual's mental health condition(s). MH Nursing is a fast-paced field combining "hands-on" (such as wound care, phlebotomy, medications management etc) and psychological elements of nursing. There are a broad range of roles for mental health nurses, such as Perinatal Mental Health, CAMHS (Children & Adolescents Mental Health Services), Acute Care, Community Psychiatric Nursing (CPN), Crisis Intervention & Home Treatment Teams, Psychiatric Intensive Care (PICU), Forensic MH, specialist services such as Eating Disorders, Personality Disorders, Dementia, Substance Misuse... etc
Adult NursingAdult Nurses work with people over the age of 18. As an Adult Nurse, you'll be working with patients with all sorts of backgrounds and health conditions, looking to improve the patient's quality of life. As an Adult Nurse, you can work in areas such as medical wards, surgical wards, operating theatres, intensive care, geriatric care, gynaecology/womens health, trauma and orthopaedics, general practice nursing (GP nursing), district/community nursing, outpatient clinics, charities, prisons etc
Learning Disability NursingLearning Disability Nurses, like mental health nurses, can work across the whole lifespan from children to adults. LD Nurses help individuals with a learning disability to live fulfilled, longer lives, reducing barriers to allow for more independent living and help them to meet any of their personal goals. LD Nurses can work a wide range of settings, such as individual's own homes, areas of education/work, community centres, residential centres, inpatient/outpatient areas of hospitals, mental health settings, prisons, charities etc. You could also specialise in areas like sensory disabilities, autism spectrum disorders, respite care etc.
Hopefully this is helpful in allowing you to see a small list of areas you might be interested in/which field best suits that... or just to show that there's a large scope for jobs in every field of nursing!