We cannot stipulate that a 6 year old does not understand the concepts of life / death / murder / assault, but that stipulation might be baked into the laws of the jurisdiction. In Washington, a child under age 8 is statutorily incapable of committing a crime. Between 8 and 12, there is a statutory presumption that a child is incapable of committing a crime, but that is rebuttable. The statute says "Children under the age of eight years are incapable of committing crime", and it say nothing about understanding concepts. Florida law used to allow that a 6 year old can commit a crime, then when they arrested a 6 year old, they changed the law. But, the law says
A child younger than 7 years of age may not be taken into custody,
arrested, charged, or adjudicated delinquent for a delinquent act or
violation of law based on an act occurring before he or she reaches 7
years of age, unless the violation of law is a forcible felony as
defined in s. 776.08
so a 6 year old can be arrested (charged, tried, convicted) for murder. Virginia does not appear to have any statute declaring 6 year olds to be categorially incapable of committing a crime. The question of capacity is of course a real consideration – insanity remains a defense against criminal charges.