Why are children used in war and how does this take away their Human Rights and Freedom ?

Children in Afghanistan are sent to spy and are also  messengers. There are 300,000 child soldiers in over 20 countries world wide they go into people's houses and kill other children's parents and then they take the children to somewhere like a base and brainwash the children then send them out with guns and kill other children parents and bring there children back and brainwash them.There is about 20% of children working for there priorities and there family's priorities.The jobs that are included are suicide bombers, Human shields, Messenger's, Spies, fighters and cook's  Child soldiers have lost there freedom because they don't have the human right's  to be kids and to do things kids enjoy to do in there own time