OK,
Peds NCC does exist, and at a few select places they are in high demand. But in most locations there isn't a market for full time pediatric NCC, so most end up doing pedi-CC with a concentration in NCC, which is fine if that floats your boat. From a research standpoint it can be good, although the pedi-head trauma world is already well-populated with neurosurgeons and standard trauma people. I don't know about being able to just do whatever you want after you're trained, though. If you are willing to live in a big city, and willing to do mostly standard PICU, then I guess, but remember that PICUs and pedi-neurosurgeons have been basically doing Pedi-NCC on an ad hoc basis forever, so you'd really need to justify why you're so needed (and I don't think those studies exist). I'm also not sure how just being clinically trained in pedi-NCC makes you more qualified for a grant, particularly given how small the pedi-NCC population is to start and how well-developed the pedi-trauma world is already.
An intern year in medicine does almost nothing to prepare you for critical care neurology. You can learn a lot of medicine during neurology residency if your program is rigorous, but learning critical care is what fellowship is for. The overlap between general medicine and critical care medicine is pretty narrow, anyway.
In my opinion, if you want to do NCC from a med-peds background, you should justify why you didn't do neurology in the first place, and play to your strengths. Peds-NCC is one way to do that. Another way would be research that harnesses more of your medicine background. Basically, you don't want it to look like you fell into medicine by accident, and are trying to rectify the situation by bringing yourself into NCC as a silver medal for the neurosurgical training you always really wanted. That's not what we're here for.