According to my quick search, it should be Not more than 5 years into the deaths by gun-wielding toddler surpass the deaths of 9/11.
Edit: I was wrong. Here are the fruits of my research:
Toddlers: about 21 deaths per year, would take until 2142 to surpass the 9/11 deaths
Kids 12 and below: 40 deaths per year, would take until 2075 to surpass those deaths
Children 17 and below, when counting accidents only, would take until 2029
Dude! That clothes rack is… EMPTY!!!