9 year olds wandering round with guns with no supervision at all is just asking for trouble. Kids are stupid and do stupid things, no matter how sensible you think they are, the minute you turn your back they're pushing what they can get away with. If they're not then there's something wrong with them.
Protecting a couple of $2 chickens (or pretty much any other justification someone might come up with) just isn't worth that risk.
If you had any appreciation for just how tight farm life is (on a money/time constraint basis), you would not think so.
Example, if we lost 36 hens and the rooster, we now have to pay around $100 to replenish those chicks and that assumes they are all properly sexxed (I could buy 36 girls and one boy but end up with 33 girls and 4 boys, in which case 3 have to go).
Also, during that period assuming all of the hens were of mature egg laying age, you can figure about an egg a day. It takes 5-7 months for them to reach that mature age and you can sell a dozen eggs even at that time for a couple dollars. So do the math, 180 days production lost at 3 dozen a day at $2 a pop, you've lost $1,080, not including the replenishment costs, so nearly $1,200 lost.
Even nowadays, if you are operating a small family farm, that's a lot of income to just have exaporate.
I realize in this day and age, its a strange example to most people, only reason I bring it up is its an older style of living where the younger generation had a hand in the real family concerns and grew up early. Speaking in absolutes about what you can and cannot do doesn't work when there's still a segment of the population that from my experience shows it's nothing to do with what you are handling (firearms in this case) but how the person makes good or bad choices. If you raise kids to be kids, they'll grow up to be big kids.