I do not practice in your state, but under traditional common law principles, parents are generally not liable for the intentional torts (like the battery you describe) of their children under a certain age (often 7). The common law deems children of such a tender age to be incapable of forming the requisite intent (put another way, kids will be kids). For a clear answer to this piece of your question, you will need to consult a personal injury attorney in your area.
Parents CANn be liable for their own negligence. If, for example, the six year old's parents were present for the incident and saw their son waving his helmet around in the air in a menacing fashion before he struck your son, they might have been negligent for failing to stop him. A personal injury attorney can help you assess whether you have a viable independent theory of direct negligence against the parents.
I hope this helps.
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