As a responsible firearms owner you must restrict access to your firearms from unauthorized use. There isn't a child alive that isn't curious or exploratory. There isn't a child alive who follows all of the rules. Please don't rationalize yourself into a tragedy.
Safe's don't have to be expensive. In its simplest form, a pad lock behind the trigger that prevents the trigger from actuating or a cable through the action and out the magazine well can be used to protect your firearms.
Stack-On makes some affordable safes that can be had for a few hundred dollars. Now please understand, these are designed for keeping little fingers off of your firearms. For theft protection, you will need to step up your game into something more robust.
The most important thing you can do for theft protection is to bolt your safe to the floor. We also highly recommend you keep a file with the make, model and serial number of all of your firearms including pictures of any accessories on them. This file will be invaluable in the event that you need to file a police report.
Complacency is what leads to tragedy. If you are not in direct control of your firearms; secure them.