I think one has to consider the actual development of a child and their ability to understand what a gun actually is and the difference between an airsoft gun and a real gun. Developmentally, a child up to about ages 6-8 does not understand the concept of the permanance of life and death. To them, life is like a Road Runner cartoon. Wiley coyote gets flattened by a truck and then pops up again. Life and death is interchangable. Clearly this would be part of my explanation.
In the same line of thought, my concern is not so much my child seeing my airsoft but seeing my airsoft rifle and then seeing almost exactly the same weapon on TV in movies, TV shows or the news. They see death being caused by a weapon, the same as daddy has at home. Therefore, there is the possibility of confusion and possibly the generation of fear, even mortal fear. If the child is aware of your airsoft rifle, then at some point there has to be a simple discussion with the child on exactly what the airsoft rifle or handgun is and the difference between them and real guns your child may see on TV or at a movie. The real danger is not the information you give your child, but the information they receive, uncensored from the idiot box.
With the Baby Boom Generation, following WW11, toy guns were everywhere. Ask your father about "Johnny 7" rifle, or "Mighty Moe". Part of growing up in the suburbs was to play war games. We knew the difference between the toys we played with and weapons our fathers carried in WW11 and Korea. I did Civil War re-enacting for 14 years. My children grew up around muskets, swords, bayonets, cannons from age of 3 & 5. They understood. I was always careful and respectful with weapons around them. They were at re-enactments and saw huge battles fought with explosions, cannons going off, thousands of muskets being fired. Men dropping like flies. Far more violent than airsoft. They understood and knew the difference.
So, whats the answer. Just use good sense. Treat the airsoft rifle with respect. Help them understand the difference between an airsoft gun and real ones. Teach them in simple terms they can understand. Recognize learning opportunities when they arise. I think by asking the questions that you have already asked shows you're responsible. A good first step. Congratulations.
"Let us cross the river and sit beneath the shade of the trees on the other side."