As a parent and a daycare provider for young children, I can totally see how this could happen. You get busy folding laundry, taking care of another kid etc etc and you do get sidetracked. Should it happen? No, but it does. Accidents like this are liley never to be repeated by the same person and in this case I am can almost guarantee the adult in charge will be more attentive.
Yes a 4 year old SHOULD know better, but kids lack the impluse control that we as adults have (and even some adults lack this skill). I think in his mind his thinking was logical, probably to small to turn the tub on, to short to reach the sink but just the perfect height for the tolet...in his young mind he was just giving the puppy a bath. Our flushers on the toilets are easily hit and flushed. The boy DID tell his mom, so he was aware that it was a bad thing, but I honestly don't think he was old enough to see the future of that puppy going down the drain. At 4 they just don't have teh capabilty to think as we do as adults. The mom may not have put 2 and 2 together, she may have been busy with anotehr task and didn't see the boy with the pup...a lot of things in this story are missing and it isn't fair to judge the mom or the boy without knowing ALL of it. I do believe the boys intentions were not malice, otherwise he probably wouldn;t have said anything at all.
My daughter was not quite 2 yrs old, we were both in the bathroom. I was right beside her and watched her drop a toy into the toilet, before I could react she flushed the toy. Had to call a plumber to come and remove the toilet, find the toy and replace the toilet. What made it even worse was that it was Christmas eve lol, getting a plumber at 6pm christmas eve was a fight, needless to say the bill was HUGE lol. So things DO happen, even with an attentive, decent parent right there, they just do.