I have the greatest admiration for the grandmother who appears to have saved her grandson's life, losing hers in the act - a truly selfless act, but the "suddenly, they fell" description is very vague and doesn't really explain much.
The original poster to whom you replied, was correct. How did they manage, together, to slip so easily into the canal?
There is an element of 'avoidance' here, such as not allowing herself or the small child to be anywhere near where they might place themselves in danger.
But then again, but for the grace of god, who doesn't make an error of judgment once in a while. Most of us do, but usually without disastrous results.
Hindsight is a wonderful process.
Sad for the grandmother and the family. Hope they ensure the child grows up to remember her sacrifice.