a 5 year old right. no pointless hyperbole here.

The LAW still applies, the maker of the firearm manufactured from that kit MUST apply the serial number.
I don't have to buy a kit from them or any other source of 80% components. If I have the skills (which I don't, neither do most people, particularly a 5 year old), I can build my own from scratch, legally, as long as I follow the law. The law already covers all of your arguments!
It is not a loophole, it is a right. I can make a stick, I can make a knife, I can make a sword, I can make an ax, I can make a halberd, I can make a cannon (which I did in high school. A fun little 14" bronze popper that actually fired a projectile) and I can make a rifle, all legal expressions of my 1st and 2nd amendment rights. All legal, because I have the right to defend myself from not only the low-life jack-wagon that steals your wife's .22 rifle, but also from you and your low-life jack-wagon demoRat politicians that think I don't have the right to defend myself with arms I have made myself.
PS, when you do teach that 5 year old to build a kit gun, be certain you permanently apply your serial number so that you are Legal!