I haven’t seen the show, but my guess is that the script numbers the kids in order of appearance – because it would be really confusing to get stuff ready if they weren’t numbered in order of appearance. Imagine reading the script and seeing a first mention of the kids like:

Ghoulish child #2 darts across the hall and disappears. You wonder, “Was there a #1?” Then you see more ghoulish kids on the pages: 4, 7, 1, 5. Are there numbers 2 and 3? 6 or 8? How many costumes do we need, and are theses kids going to appear together? Were some cut? Did the script editor forget something?

If they are in order of appearance, then the kids with bigger/speaking parts might get higher billing, but they wouldn’t get earlier numbers since non-speaking/smaller parts appeared earlier.