I think you need to think of the definition of 'child'. Are we talking an age, or a maturity? I know a few people who literally act and think like children, even though they're at an age where they should be thinking of more serious stuff. On the other hand, i know a few kids who seem like they're eighty.
There is no 'in general' answers. There is no black and white with this. There must be rules involved.
If the adult had bone marrow that was only compatible with my daughter and i had a choice between shooting him or a little kid across the road, I'd shoot the kid, because it would let my daughter live. I'd be grief-stricken for life, but I'd do it.
If it was the choice between a random man and a random child, I'd shoot the adult, because the child hasn't had a chance to realize his potential.
And, of course, there was no real mention of method (besides the gun). If i had to look the kid in the eyes as i prepared to slit his throat, seeing the terror in his eyes, i wouldn't be able to do it. Same with an adult.