These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Family-Unfriendly Aesop: "Kids, if you get into trouble with an armed assailant, don't bother calling the police. You can dispatch him on your own."
Considering they were successfully kidnapped and put Emily in danger another one could be: "Support your kids so they don't feel like they have to prove themselves by getting into dangerous situations".
"Funny Aneurysm" Moment: In the original film, when the titular ninjas were returning home from their summer break with their grandfather, they joked about murdering their teacher, though they never made any indication that they would actually do it. Long after the release of this movie, there have been several high-profile school shootings, Columbine being the most famous. No, this movie was not the impetus for the killings.
Sequelitis: Three sequels and each one did worse at the box office and sank lower in critical reception.