The collection of protection fees for several children is generally extortion and will not be convicted and sentenced. Unless your son uses violence or threatens to use violence on the spot, such as hitting each other with more serious violence, threatening with a knife, etc. You said they handed over 50 yuan and returned it to 10 yuan. It should be that the process is peaceful. General intimidation and coercion conform to the characteristics of extortion, but they do not reach the severity of robbery.
Good luck!