Plot hole: The army showing up at the end of the movie is wrong on numerous levels. 1) Eugene wrote and sent out the letter that day. I find it hard to believe the mailed letter got to the army in just a few hours time. 2) How did the army know to show up at time square? When Eugene wrote the letter, nowhere did it say where to meet. Plus the boys didn't decide to go to time square until the last minute. 3) since when is the army deployed to battle monsters based on a letter that was written by a child?
Plot hole: In order to get rid of the monsters, Phoebe needs to read a text from Van Helsing's diary in order to summon the vortex. As Dracula approaches her, she becomes scared and misses on some words (the Scary German Guy is helping her read since she is 5 years old and she doesn't repeat some of the words he says). However the vortex still shows up to get rid of the monsters.
Plot hole: There are numerous problems with the final scenes in this movie. First, it's inconceivable that these kids are out at midnight at all; the boys are only 12 and two others are 5. Second, it took several minutes, possibly miles, to get from the house on Shadowbrook Road to town square, but the vampire mistresses make it there in mere seconds. Third, the sign in the store window where the two bullies are hiding said closed, so how did they get in? Fourth, Patrick grabs slack from the mummy's wrist but when the arrow pulls tight, he begins unraveling from the foot.