I think spanking/bdsm should only take place between consenting (and I do mean consenting, forced punishment is abuse plain and simple) adults and I don't particularly like reading about it in stories that contain children, especially the ones that contain younger children. Spanking fiction is sexual fiction and should not contain any character participating in those activities under the age of 16, although I have less of a problem with "school" spanking fiction about teenagers provided there is no sexual element to it on the part of the adult (if adult it is) adminstering it as I'm sure any of us who are over 40 remember it being around at school (if infrequently towards the end). I have no problem with anyone of the age of 18 or over participating in these activities of their own free will.
I do know with the stories we are talking about fantasy, and that many of the writers maybe fancy themselves in the position of the child rather than the adult but I still find it makes me uncomfortable. I don't have much problem with it being children (so long as there is no sexual activity) in historical settings as, let's face it, that can be a very accurate representation of what went on in ages past but my own preference is not to read spanking fiction involving children, other than the odd school scenario. Parental discipline stories are a squick factor for me so I tend to avoid them.
All the people I have met in real life and online through the bdsm scene have been at the youngest in their early twenties, an age at which one can hopefully make reasoned choices about sexual preferences and partners. I don't agree with corporal punishment in the home (other than maybe a light slap for very young children, you can't exactly explain things to them at that age easily) for children of any age and frankly the idea of fathers or mothers using "ritual" punishments with implements on their children makes me sick. Those of us who are submissive/masochistic know very well how much some of this stuff can hurt so to do that to a child is horrific. A far better punishment is to deprive the child (even older teenagers if they are at home still - you abide by your parents rules in this situation) of something they really love for a while, a little psychology goes a lot further than a beating. The lack of a sexual motive in domestic punishment makes no difference in my opinion, I should hope that there is no such motive, the possibility that someone could get a kick out of beating their own child (even if they are 18 or over) makes me feel sick.
I recall a debate on a TV programme once about smacking children and someone pointed out that how would you like someone a lot bigger and stronger than you to hit you as a punishment (when its a punishment for, say, hitting a sibling, this becomes positively ironic and sends the message that violence is acceptable from the parent but not from the child, hypocrisy might be a better word for it), often with an inappropriate level of force. Beating children is wrong, and beating is what it is whether you be a consenting adult participant or a helpless child, if you have a problem with that word then you are in the wrong game my friends (to quote Josepine Scott - bsdm fiction writer and devotee).
EDIT - please bear in mind whilst reading this that I am from the UK, age of consent is 16 and we are considered to be adults at age 18 whereas I understand other countries (particularly the US) have different age limits. Oh yeah, and I'm female just in case anyone didn't realise, LOL, I noticed a comment earlier about most replies being from

males