NAIROBI - More than a dozen senior commanders and officials who children say recruited them as soldiers in South Sudan should be investigated, Human Rights Watch said on Tuesday.The United Nations says 16,000 children have joined armed groups since South Sudan's civil war erupted two years ago."It's the brutal recruitment that is the most heart wrenching," Skye Wheeler, the report's author, told the Thomson Reuters Foundation."Forces come through their village and grab them and force them into fighting. It's an absolute negation of their basic rights as children, but also as people, not to be treated just as cannon fodder."South Sudan was plunged into a civil war in December 2013 when a political crisis triggered fighting between forces loyal to President Salva Kiir and rebels allied with his former deputy Riek Machar. The conflict has reopened ethnic faultlines that pit Kiir's Dinka people against Machar's ethnic Nuer people.A peace deal was signed in August but the two sides have repeatedly accused each other of violations.