The problem with some rifles is that the chambers are cut tight or left in a semi rough state.  In time, the chambers will self-polish out threw live fire.  Also, some rifles are supplied with extractor and springs that are less than stellar.  As stated, the O-ring (and other two) is band-aid that gives the extractor the extra needed tension to allow the rifle to break in.  In a few hundred rounds, you will be able to pull the device and the rifle will run without it (unless you have a bad extractor or spring that needs to be corrected/replaced).

Since, I'm a huge KISS fan (Keep it simple stupid), the O-ring cost $.10 at any hardware store, as where the other items are hovering around the $10 mark. Now to add since Colt has come out with a extra power spring improvement for the M-4, the reason behind this stronger spring (black insert marker) is the rifle is designed to shoot what is considered non-standard issued ammo (heaver bullets using slower powder which causes higher barrel pressure at unlock), and this stronger spring assists the extractor in pulling the case as it is under higher surface tension during unlock.

Simply put, if the rifle/parts are in working order, there is no reason for the added items.  Where the band-aids come into effect it that allow the rifle to work threw the problems, instead of needing to be smithed.

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In regards to adding items to a firearm to make it child proof, there is no such thing.  Trust me, give a 6 year old enough time, and he will crack a Gun safe if you put something in it he/she wants. The best that you can do with your kids is teach them firearm safety from the beginning, and make sure it sticks. Also, instill in them that they are not only responsible for their own actions, but also the other people around them. Not only is this important for gun owners children, but to the other children in the neighborhood as well.  When the time comes for your children to take hunters safety, Round up as many other children in area that you can find and in role them as well. You will be surprised to find that even the most anti-firearm families will favor the fact that although they may never have a firearm in the house, there children may come a crossed a firearm, and have firearm safety training if/when the time arises.