When the main cast is trapped within the souvenir shop surrounded by zombies, the zombies surprisingly don't really make much of an attempt to break into it to get them despite frequent noises (including shouting and singing) emanating from the place. Towards the end, it's revealed the zombies become docile when music is played, meaning that all those points where Dave and Miss Caroline played music for the kids, doing so actually helped keep the zombies from attacking.
Dave smashing his guitar over McGiggles’ head is symbolic foreshadowing of his personal growth. As the movie goes on, he slowly matures and learns to handle kids better, as well as move past his fear of children. In order to do so, he needs to let go of his baggage, which is symbolized by the guitar.
It might seem inefficient for Miss Catherine to tell the children that the whole zombie scenario is a game, but it makes sense. Kids are easily frightened, unstealthy, and stubborn in the face of danger, and are prone to freezing up or throwing a tantrum when they're scared, which is a quick way to attract zombies attention or stall the group long enough for them to get eaten. If the kids think it’s a game, then it’s only a matter of getting them to cooperate in the same way both the kids and Miss Catherine are used to.
Miss Catherine wears a bright yellow dress, and yellow is a color symbolic of joy and fun, which is fitting because she’s a kindergarten teacher whose really good at her job. Dave wears a lot of dark colors, not only because he’s a dour asshole, but because he’s upset that his band left him and never let it go. By the end when the class is in quarantine, he’s wearing all grey, showing that he’s lightened up a bit.
Also, throughout the movie, Miss Catherine, David, and McGiggles all have blood on them at some point, whereas the kids have nearly none. Miss Catherine’s goal is to keep their innocence intact, and she succeeds at it with David’s help. The adults all have it on them because they do some pretty insane things, both before and during the movie.
McGiggles wears a bright green suit, and green is usually associated with envy, but is more rarely used to symbolize lust and greed. Throughout the film, McGiggles is the only character to never demonstrate any positive qualities: he hates kids but is obsessed with the spotlight, hates how he’ll never be anything more than a kids show host, and only lives to have sex with moms.
As the army begins to set up a perimeter, one soldier asks another whether they are dealing with are the slow kind of zombies or the fast ones. Stating not only that there are fast zombies, of the 28 Days Later variety, but that this is not the first time they have had to deal with an outbreak.
The virus was the result of the US army trying to create super soldiers. If there have been outbreaks prior to this one than that means the army knows about the risks... and have simply continued their research and experiments anyway.
It is possible that this is the first outbreak, the soldier asking this question is merely Genre Savvy reacting to a briefing that boiled down to 'we are quarantining a zombie outbreak' and fast zombies don't exist in-universe. This still indicates the US military, on the whole, isn't Genre Savvy as they chose to create a zombie virus despite zombie media existing in-universe, but this makes it less egregious.
McGiggles probably abandoned his camera man and crew to save his own neck. He tries to sell out Caroline's class and Dave multiple times. It is very likely he did the same thing to everyone else he ran into
On that same topic, Miss Caroline's class couldn't have been the first ones who tried to get into the gift shop. How many people did McGiggles get killed by locking the door and refusing to let anyone else in?
Among the zombies, we see PLENTY of children, all of whom were probably either there with their families, or on class trips themselves. In other words, a ton of other groups in our main characters' situation, except they all failed to protect each other. Imagine being on a pleasant family or class trip, only for the day to end in horror.
It’s highly possible there could have been other people who survived the zombies other than Miss Catherine and her class, David, and McGiggles; they could have been hiding in the woods, or running from the zombies. Unfortunately, we'll never know, since The US Military blew up the park, instantly killing anyone, zombie or not.
