The corn looked good, so I suspect there was rain, but the fireworks (rockets) were always out and ready, and the sand (or ashes?) was spread out on the path everywhere they went. A little rain would have ruined both of those hair-brained plans.
There were maybe a total of 5 distinct days depicted in the movie and you post a comment about not seeing it rain?
So one cam guess that they kept the firworks dry.
You witnessed the father putting down new sand. So yes we know that they had a system for replacing the sand.
Even without all of the above, if things like this bother you maybe movies are not the thing for you.
The stupid upward facing nail in the middle of a stair tread that did absolutely nothing for the plot is a far more aggregious clump of BS than what you are worried about and you never even noticed it.