The image of a two-year-old being crushed and killed by a speeding big rig is beyond horrific, and while the passing of any child is absolutely horrible, the fact that Gage is as young as he is does make it exponentially worse.
Without explanation, this could easily be interpreted as the production not wanting to cross a line, and making a choice that the death of a preadolescent is more tolerable for an audience than that of a toddler.
The biggest issue here is that the movie has seemingly pulled the razor sharp teeth out of what Stephen King has called his scariest book – one that he didn’t initially want to publish because he felt it was too disturbing.