Hi Shane2150,
I think you are in a great position to do Child Neurology, as you will begin the pediatric residency years soon. I would not advertise to your program, i.e. don't talk loudly about your plans to do Child Neurology, but focus on doing well in your pediatrics training as it will help you be a better pediatric neurologist, and you will want to get letters from your pediatric residency attendings. If you are at a big program, especially on that is affiliated with a Child Neurology program then I would ask around, anonymously at first to see what the options are. Bigger programs can absorb the loss of a resident much easier than smaller programs. Alot of Child Neurology are/will open up for PGY-3 training in Child Neurology, just check out the San Francisco match page and they are always listing opening for PGY-3 child neurology positions, alot of them are at surprisingly prestigious places, so fear not, you will be able to secure a good to excellent PGY-3 Child Neurology spot somewhere at least for starting 2009.
I would guess that you willl need to get a PGY-3 child neurology spot outside the match, which you can do easily when the time comes. Basically keep your ears open as to where a spot might/is open for a PGY-3 resident. Alot of programs, for various reasons, may lose a resident who was committed to Child Neurology for PGY-3 2009. Be glad that you have secured a pediatrics residency. The sticky part for you won't be finding the PGY-3 spot, many PGY-3 Child Neurology programs will look at you favorably having completed two years of pediatrics residency. Technically, right now, there are a large number of open PGY-3 child neurology spots for 2009, and you are well within your rights to sign an outside of the match contract now with a PD of a child neurology program. Go to the San Francisco website, then you have to fax your application, plus LORS, plus usmle scores, to a PD, there are programs that would give you a contract now for the PGY-3, PGY-4, PGY-5 years, especially since you are set for pediatrics residency, the only issue is a legal one regarding whether you can sign a contract for PGY-3 child neurology when you (I would suppose) have agreed to three years of pediatrics residency training. You will need to get your current PD to modify your current contract if needed, and your current PD will need to write a letter to the child neurology program director saying that you were a competent resident, etc . . .
The hard part will be how to gracefully excuse yourself from the PGY-3 pediatrics year. I would focus on doing an excellent job on the pediatrics residency portion, at some institutions pediatric neurology residents are often the chief pediatric resident in their final year of study, so keep up a reputation for excellence. I would keep mum about your decision to pursue pediatric neurology until after PGY-1 year. The worst case scenario is that the pediatrics program could make your life very "difficult" if you decide to defect to child neurology.
I would do a good job during PGY-1, then spring the news later maybe 1/3 of the way through PGY-2, because at that point, hopefully, the attendings will have a good view of you as a resident and won't want to make your life to difficult, and it gives them a headsup that they will need to reorganize call schedule or advertise for a PGY-3 resident position. If your peds program is massive, like 50+ residents, then this is less of a problem than at other places. If they are very academic/research focused then they might help you a small amount with the transition to child neurology, but wait to let the cat out of the bag until the moment is right . . . if there a child neurologist on faculty where you are at I would talk with them to get them on your side.