Getting a gun, pressing it against the chest of a 4 year old and pulling the trigger
Versus
Getting a knife, pressing it against the chest of a 4 year old and pushing it deeper
What's the added difficulty here? Yes, in general you are correct but in this scenario it really wouldn't have made a blind bit of difference. A 4 year old's capacity for self defence against an adult is basically zero, this one's chances of getting to safety was basically zero. Even if you removed both guns and knives from the equation, they would have just used something else.