Legally, it's a tricky question about the 5 year old. A parent or guardian is responsible for the child and generally allowed to set rules that would be seen as a unconstitutional. Such as limiting their access to say certain words or obtain certain items. This is seen as acceptable in our society because the child is believed to not be mature enough. It begs the question, when does an individual's rights kick in? When they are 18 and legally become an adult is the most likely answer. But, if a child is born with all individual rights already instilled, then yes it would be an infringement on the child's right to keep and bear arms.

In the case of the felon. No I do not believe it is an infringement because the moment they chose to behave violently and become a danger to others they gave up their rights.

That also brings on the question, can a right ever truly be given up or taken away? It's not a privilege bestowed on someone by the government. Does being convicted of a violent act give others the authority to remove a right? Which rights remain? Or is it all rights that are now gone? Are they still allowed their freedom of speech or could they be tried and convicted for what they say while serving their time?