Really, beyond the hyperbole, the straw man arguments, the deflective strategies and the chest thumping, shouldn't the parents be the ones to make the decision?
If you think your kiddo is ready for a slip joint with no safety device, then buy it. As a parent, you are the on gambling with your kid's safety. Get it right, and good for you!
If you feel that your kiddo needs that one tiny bit of extra protection that a lock might give in case as a youngster he/she has a brain f@rt, get one with a lock. It goes without saying that a locking device won't prevent accidents or dumb behavior, so one should prepare for that as well.
Either way, its on the parents to have their children's safety and well being in mind. A parent that puts a knife in the hands of a child is responsible for the results, good or bad. I am not in favor of the nanny state mentality and know that every child is different, as will be the instruction they get from their parents on how to use a knife.
I think as long as a parent can look at a gash, scar, bad cut or slice, or anything else that a child might do or have happen as an accident and immediately own up to friends, family and spouses with a "hey... sorry, that's on me!" regardless of which type of knife was involved might be a good way to look at it. They should approach their proud proclamations of damage and its results with the same zeal that they do if a knife is purchased and all goes swimmingly, when they will no doubt take credit for their fine parenting skills.
This thread seems to have devolved/evolved into more of a commentary on societal right or wrong rather than a discussion on which knife to buy for a child. The child was left a couple of pages back...
Robert