3 characters moon their bare buttocks at a principal in one episode, but it's very brief.

A naked baby picture of a character is shown, only their bottom is briefly exposed and it's nothing sexual.

Various female characters wear revealing clothing over the course of the series.

A character witnesses another character undress while hiding behind an electronic couch, but it's played for laughs, nothing is shown.

In later seasons, butt cracks are briefly seen.

Mild innuendo

On the "Journal" episode, sexual innuendo was mentioned in the journal, but the words were never read aloud.

One character has an ongoing crush (which borderlines on obsession) on another character.

A teacher character is strongly implied to be a homosexual

One episode showed a nudist beach, but only a couple buttocks are shown. Other episodes are similarly revealing.

There's a running gag where Helga punches Brainy in the face, but it's played for laughs.

There's some bullying between certain characters.

Arnold reads a grave and says he can't read the rest hinting the world "hell" is written on the tombstone, as the first two letters are seen. (Hell isn't necessarily a profane word unless if it's used like one)

"Crap" is used in some episodes. Nothing big. in one episode the British profanities bloody, nit and bollocks are used. name calling Moran, football head, losers etc. Helga and her father both say "criminy", which can be used as a euphemism for taking the Lord's name in vain.

Helga's mom is strongly implied to be an alcoholic. The smoothes are cover ups. This is supported by the fact that she constantly slurs her speech and passes out frequently on the couch.

In the episode "Part Time Friends", there's a scene where Arnold has a nightmare about he and his friend Gerald arguing inside a bus while they're old men, only for Arnold's Grandpa to mock them both, and we cut to the old, withered, zombie corpse of Arnold's Grandpa where he says "Well, what did I tell ya, Shortman?" before laughing in a really creepy way, only for his jaw to crack and fall off his skull. This scene is extremely scary, and can be pretty disturbing for children.

Some episodes contain a ghost and a headless cabbie which might be too scary for smaller children.

Helga's dysfunctional family might upset some.

Characters go through serious and sometimes emotional situations like someone's child being sent away because of an incident at war, rescuing a bunch of pigeons, bullying from taller kids, stealing, and the sad story of what happened to Arnold's parents.