IME, peers applying for peds-neuro internship and fellowship positions have described it as seemingly more competitive than my friends who have applied for similar adult neuro positions. I’ve similarly heard that there is high demand for boarded peds neuropsychologists.
To give you an overview of how you’d get from A to B: After HS, you’d want to get a 4-year degree (most likely in psych), during which you should get involved in ongoing research that interests you. After graduating, you would then apply to clinical psychology PhD programs. The PhD should provide you with strong generalist (i.e., scientist-practitioner, assessment and treatment across the lifespan) training, and will, ideally, provide you with full funding (i.e., tuition/fee waiver, annual stipend/salary). It would be ideal if you could find a doctoral advisor (professor who is your primary mentor) who works predominantly in pediatric neuropsychology. After ~4-5 years of doctoral training, you will apply for a 1yr internship, which will likely be more targeted and specialized than your pre-internship doctoral training. After completing internship (and defending your dissertation), your PhD is conferred, after which you begin your postdoctoral residency/fellowship, which, for peds neuro, will be (at least?) two years.
tl;dr - peds neuro can be competitive but lucrative; timeline = undergrad (4) + PhD (4-5) + internship (1) + postdoc (2) = 11 years of higher ed