I don't get washing out someone's mouth with soap. Surely there are things in soap that shouldn't be in the mouth, and potentially swallowed? It seems like a huge safety hazard to me, as well as just plain stupid. I know it's symbolic thing, but there is something about forcing soap into a child's mouth that just makes me think "that's wrong."
Ivory soap is non-toxic. But it does burn like hell. Believe me I had it done once or twice.
I doubt many families make a point of having the perfect mouth washing soap available, many may just use whatever they find, which scares me.