As an adult, you can be put in jail for violating the rights of another adult. If you destroy something that is my property, I can get you locked up and I can sue you for reimbursement. If you punch me, I can get you arrested.

Children enjoy quite a bit of immunity from these repercussions. If one 6 year old deliberately breaks the toy of another 6 year old, he really won't face any repercussion, other than what his parents impose on him. He won't be arrested, or sued. (Though parents of the child wronged can sue parents of the child who did wrong.) Parents are held responsible for restraining their children from violating the rights of others. Bare in mind that a child of 6 is quite capable of killing another small child. So the law has traditionally deemed it wise and appropriate to leave parents free to restrain their children using the means they chose, but not permitting overt abuse. (Overt abuse being judged based on the broad consensus of societal norms.)

The argument that a child should have the same right to freedom from physical coercion that an adult enjoys is failing to recognize that the child already has rights and freedoms that adults don't enjoy.