Rate could mean over time or percentage of. I think OP was thinking of time, in which case your wouldn't need the total number of schoolchildren. Just divide the population of billionaires by the number of schoolchildren that die by violent crime per year.
If I'm reading the hypothetical right, it's saying what if in a year, the numbers were reversed... 323 $hootings with 69 deaths, versus 1 school shooting with 1 death.
Would gun laws change, would tax laws change? Etc etc. Would minimum wage change?