I don't know about tazers but as a parent of 3 almost grown children I can assure you there were plenty of times that I thought a couple of those shock collars we used to use to train hounds would have been real handy.
as far as it being OK for cops to tazer people in general or kids specifically? obviously it would depend on the circumstances and were less drastic means considered.
personally I think I'd really hate getting tazered, but would rank it considerably above getting shot. I'd prefer that cops have as many other options as possible before they have to resort to shooting or killing me or anyone else.
at least with the tazer you can file a complaint and have your day in court (if the cop was out of line). while if you get shot they may still have the hearing, though 'winning' isn't likely to be near as satisfying for you.