The child stuff, is merely a parenting issue. Otherwise, 2A is more about adult citizens as I see it. A "child" or "teen" does not have the right to buy beer, cigs, etc, etc because they are not of adult age. (adult age is another discussion entirely, however for simplicity we'll stick with 18)
After becoming an adult, 2A fully applies to you, until you become a convicted criminal (gone through due process, stood in front of a judge, etc) and at that point have given up your freedom and therefore rights to 2A among others..
Shall not be infringed means exactly that. Other than the two examples of not yet being an adult with rights, or having them stripped by due process. Mental illnesses can fall under the due process as well. I know that comes up a lot, but really. When someone who is taking care of such a person or knows of a person that is mentally ill, say something or do something about it, so due process can happen and do its thing. Please for the love of whatever you believe in.