While I completely disagree with what those irresponsible parents did in leaving the toddler unattended, I also disagree with doing anything that would make the parents freak out too. Yes, they clearly need to learn a lesson, but there could've been ramifications from doing that too.
I realize that the odds of the following scenario happening are incredibly small (but possibly even greater than those of a toddler being abducted at WDW), but consider this : What if the OP had done something that made the parents freak out like say she saw the child wander off, and one of the parents had a heart attack upon learning their toddler was gone ? Would that still have been the right thing to do ?
Realize that I'm subjecting myself to flaming, and am donning the flame retardant suit inbetween keystrokes.
Again, I COMPLETELY AGREE THAT WHAT THEY DID WAS WRONG, but I also don't believe two wrongs make a right.
Now, doing what the OP did was GREAT, and to be honest if that was me I would've gone ballistic on the parents if they said anything to me other than thank you for caring about our child.
Very tempting and easy to do something like that and have a very undesirable result.
Chris