I don't think I've seen a table that rolls damage separately for a single AoE effect. Calculating 10d6 takes enough time for most people.
Granted, this doesn't affect the math for the probability of any one kobold dying, but it would flatten the bell curve for the chart of the probability of X survivors out of Y kobolds.
Just out of curiosity, did you calculate based on rolled hit points or the average? Kobolds are 1d8+0, right?
Both, and it makes a big difference. For kobolds using 1d8 hit points, their odds of surviving with at least 1 hit point range from 1/37 to 1/141 depending on the wizard's Int. For kobolds using a flat 4 hit points, their odds are 1/926 to 1/3703.