First of all, if your kid got a hold of your airsoft gun, you aren't securing your firearms/airsoft arms properly. The first step is to do that. I'm not talking about a gun safe, although that is the best option if you can afford it. There are numerous locking cases made of metal or plastic that you can get fairly cheaply.
Second, teach them the Eddie Eagle program. My four year old has been well versed in all the steps for a year now. We do drills with her every few months where I take an unloaded firearm and leave it on the bed and watch my daughter as she practices the steps. She loves it and asks to practice occasionally. In fact, on my most recent return from the range, I had dumped my stuff on my bed (everything unloaded with ammo cased and in a bag) and stepped into the other room for something. I had only been out of the room for about a minute and a half when my daughter came running up to me to let me know tha she had found a gun on my and my wife's bed. She knew the steps and followed them.
Third, I let them play with my guns...ok, well not play with them, but if I have them out after getting back from the range or if my daughter sees my XD on my belt and asks to touch it, I let her. We sit down and I safely unload the firearm and then I let her pick it up, hold it, touch it, etc. This gives me a great opportunity to teach her about the importance of muzzle control as well. I also show her my plinking targets when I get back (well ventilated cans) and talk about how powerful guns are and how dangerous they can be if you don't use them properly. She gets the hint.
I plan on taking her out with my little single shot .22 bolt action rifle here soon. She can easily handle that and it will help her understand more about guns and demistify them for her. I think that is one of the most important things about gun safety and kids.