Yes, I do partly blame her. What happened was very tragic, and going onto the ride the mother didn't know this was going to happen.
However if I were a parent, and I had a four year old, I wouldn't take the child onto the ride. I have been to Disney World at least a dozen times, and from what I read recently about the ride, there are warning signs everywhere, including audio warnings in the line queue.
There would be no way I would take a 4 year old on the ride, regardless if he met the height requirement. I wouldn't want him to feel unpleasant after the ride and then not enjoy the rest of the day at the park.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle