I think both areas have great promise, though it depends what you want to do. I admittedly don't know as much about child work, though all of the normal areas of practice apply AND some additional areas in school. Competition will include non-doctoral therapist (A LOT of them) and also school psychologists.
One of the biggest challenge working with children....their parents. There will probably be a good amount of time you can't bill for, and then more time you will spend that will get questioned when you bill it. There are things you can do to minimize non-billable time, but it is still a consideration.
Neuropsychology is a great area, though not without its own pitfalls. The path is very competitive (if you want to be ABPP boarded), and it attracts a lot of over-achievers. One of the biggest threats to neuropsychology is declining reimbursements and marginalization. Many jobs don't offer that much more for a neuropsychologist, though a boarded neuropsychologist can do quite well if they find their niche.
As for combining the two.....the most competitive. There are far fewer ped neuro post-doc fellowships, though if you can get licensed and boarded, it can offer some nice $. You'll still deal with a lot of the same issues as I listed working with children, AND you are much more likely to be sued and/or dragged into court if the parents get pissed off.
Any of these routes can yield a great career, though you can still struggle if you don't get quality training, attend the right programs, internships, post-docs, etc. This doesn't mean you need to attend a "top" program, but everything needs to be APA-acred, and you need good mentorship throughout. These areas have a lot of generalists who get into positions well after they are licensed, and they aren't adequately trained. It can degrade the field and also increase your liability. I see it most in neuro, but I am sure there are people who work with kids who didn't have previous experience and/or haven't seen a kid since early on in graduate school.