If the child is young enough, they may really just not quite understand - I know a couple of times I tried to put "food" in the jar and keep them (I think my parents must have released them when I wasn't looking, since I don't remember anything dying), because I didn't really know what they ate and didn't have much of a grip of consequences. When I was six or seven, I was helping to clean out the turtle tank at daycare and accidentally killed the little goldfish that lived there, too - because I was taking them out of the water for short periods to see what would happen. I was horrified when I realized I had killed them, but when I was actually doing it, I didn't really think it would happen (I also had no experience with death - my grandparents were alive, my great-grandparents had either died before I was born or when I was an infant, my aunt, uncle, and cousin had died in an accident, yes, but when I was a year old - I wouldn't even lose a pet until I was nearly 14.) I had a magnifying glass that I played with, looking at plant matter and insects/arthropods, but I never burned any creature, just leaves. The concept horrified me. I think the difference may have been that I had been burned and knew it hurt, but I'd never felt what it was like to need to breathe and not be able to (I was a swimmer, but I wasn't yet diving deep enough to forget how long it would take to get back to the surface).

Now I study biology for real and would know exactly what to feed a specimen, and don't need to wonder about what happens to fish when they're out of water because I ... have a book that has all the relevant information, oh god, animal phys. So cool. So many tears.