Unicef define abuse as any deliberate action with the potential for physical harm. Soap is a poison when taken internally -I think that forcing a poisonous substance into your child's mouth is a high risk action fitting the above definition.
The following govt site lists simple soap as poison and that children are more susceptible than adults: http://www.healthline.com/health/bub...oaps#Symptoms4
You may not intend your child to swallow any of the soap, but if its in their mouth they will swallow some of it, and if they're distressed then there's an even greater likelihood they'll swallow it.
I find the idea that people still think its okay to 'wash a child's mouth out with soap' in this day and age quite upsetting.