None.

Very mild cartoon violence throughout this U-rated film.

Probably in order to save their own skin, one of three trolls makes up a lie to the Snow Queen about having captured a flying reindeer for her. And thus the reindeer is kidnapped.

A large bird is captured by multiple humanoid pigs in order to get cooked, but gets saved later. There is a very short chase sequence in which a restaurant gets destroyed, and a car falls down from a cliff. Sounds of the car getting destroyed can be heard in the background.

Sleeping girl and bird fall down from a flying bird and land into ice water. They get rescued by two humanoid walruses on their boat.

Two characters get imprisoned by said walruses, but are saved by a flying bird.

A bird is freezed into ice, but is saved later.

Two humanoid penguins are trapped inside a bubble.

The Snow Queen utters "fools" once.

During the "boat scene", a female humanoid walrus tends to give her male partner a bunch of mildly insulting words like "useless" and "spineless".

None.

Dimly the flying reindeer is locked in a cell next to the Snow Queen's ferocious reindeer, which constantly tries to break into the his cell and eat him. Dimly is shown to be afraid throughout.

Some very young children may find the Snow Queen to be kinda frightening.

The above mentioned scenes are basically very mild. This film is rated U in the UK anyway, and that means everybody can watch this film.