I have heard that there are chemicals which are toxic in alot of bar soaps.
Also I dont think it is used so much any more in the US I am not sure about other countries, but Lye was an ingredient in soap that caused many horrific child abuse stories with parents washing the childs mouth out. Lye has even caused deaths.

I am by no means an expert on soap, but I do caution you to research everything in the soap before you would use it.

If a mouth washing is needed perhaps brushing with baking soda, and timed requirements as to how long to hold it in the mouth before spitting. I can not imagine anyone enjoying the taste of plain baking soda.