I agree with you 100%. I've raised 7 kids myself and sometimes they do stuff you'd never guess in a million years they'd be able to do, at a young age. It doesn't mean they're bad or even able to bear the brunt of the responsibility for their actions at such a tender age, it just means they're a heck of a lot smarter than adults think they are.
When my now 40 year old was 2, she pulled a chair over to the door, unlocked the dead bolt, then unlocked the doorknob lock and "went for a walk" down the sidewalk. Had you asked me seconds prior to finding the front door wide open if she were able to do that, I would have said hands down, 100% no, never.
Unless LE give me a reason not to, I'm taking these parents at their word that the gun was secure, and somehow this little dude figured out how to UNsecure it.