With all due respect to the dead lady, and the little kid, if this was an airgun (I am ignoring the "rubber pellet" part now), it must have been loaded, cocked, etc. No 5 year old can cock a springer (unless this one a little Supergirl in disguise). Most 5 year olds have problems lifting one! Most will also have issues in pulling the trigger of an IHP, SDB, or similar Indian AG. Unless the gun has a hair trigger. But then, hair triggers are usually 2-stage (AFAIK), how many 5 year olds can go through the first stage?
It might, on the other hand, be a PCP. Yes, a shot with the PCP can kill. Say it goes through the eye into the brain. But...... even a PCP is a mite too heavy for a 5 year old to pick up!!
Likely scenario? Looks like a loaded & cocked gun left out carelessly by a dumbo. This kid somehow gets her finger on the trigger, and her mom is unlucky enough to be in the line of fire! Sad, really. But an accident, however unfortunate! Sadder still is the fact that this might lead to further curbs on the proper users of airguns.
On the other hand, the whole story, if you include rubber pellets, looks either like journalistic ignorance, or as they say - a put up job.
And Brihaji, Renjith is right, the stun guns don't deliver rubber pellets. The rubber pellets used by cops for riot control are fired from something which looks like a shotgun (based on what I remember from the curfew days in 1991) with a huge rotary "magazine" (or is it a clip?). Something like the 40 mm gun on this page:
http://people.howstuffworks.com/riot-control2.htm . Those pellets should not be lethal, unless fired at really close range and hits somewhere critical.