At that age, I don't believe children really understand what a real gun can do. I don't even think they fully understand the concept of death yet. I feel that it may be possible that playing with toy guns, especially where they shoot each other, may cause children to lose a healthy respect for guns and the damage they can do.
Of course they don't know what real guns can do. They're kids and will learn as they grow.
If a little girl has a doll that wets do you think she understands what fills a real diaper?