'A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed.' (United States Constitution, Second Amendment).
Unless 5-year-olds are now inductees in State Militias, there is no reason for them to have firearms. Of course, at 5, if *I'd* been offered the role of IRL GI Joe - well, the consequences wouldn't have been too pretty for the neighbourhood cats.
This can also, note, be extended to adults: the right to bear arms (not necessarily FIREarms) is directly linked to being active in a State Militia. Basically, anyone who agrees with both the statement 'Support our Troops,' and Heston's famous 'from my cold, dead, hands,' is, at best, schizophrenic in hir understanding of Constitutional law. <<--- not relevant, I know, but puttin' it out there...