Just because we don't run to slap the term "child abuse" or "oral rape" on it does not mean that we are stocking up on Ivory, ready to "force-feed" our child soap. Personally, I reserve the term child abuse for instances that warrant such an extreme label. My issue with some of the responses in this thread have to do with the choice of vocabulary. My God.
Some people obviously deem it child abuse and its their right to have that opinion, but I can understand why that shocks people who had it done to them as children and do not consider their parents "abusers". What surprises me though is the nostalgia some commenters seem to feel for having soap put into their mouth as children.
The UN Convention on the Rights of the Child views it as corporal punishment and sees it as "invariably degrading" treatment which children should be protected from (p. 4, III., 11.), so it's not only a few people on here who view it as unacceptable.
My issue here is with people making the matter of soap in the mouth about them and not their children. It's about the child's wellbeing and nothing else.
If you can't be bothered to teach a child why swearing is wrong with words and have to reach for the soap, you need to have a look at yourself.