Why is the cartoon kid exited about Undertale?

Maybe it's the unique characters and storylines in Undertale that get the kid excited.

Fanfiction about abuse in the context of Undertale's Kid Sans is often a very dark and disturbing concept. It might involve scenarios where Kid Sans is put through emotional, physical or psychological harm within the fictional world created by the fan writer. However, such content goes against the generally positive and friendly nature of the Undertale game.

It could imply a cartoon where a cat has exited a scene or location. But without more context, it's hard to say for sure. Maybe it's related to a story or game?

No. The Undertale community is mainly about positive things like friendship and hope. Abuse of any character, especially in a fanfiction, goes against the values of the community.

Maybe it's to teach kids the dangers of playing with fire in a visual way.

Maybe the kid lost an important soccer game or got injured while playing.

Often, a cartoon girl seems exited when she has wide-open eyes, jumping around, and maybe making excited sounds. Maybe she's achieved something great or is about to have a fun adventure.

The kid comics version of Undertale often has colorful and simple illustrations that are easy for kids to understand. The stories usually focus on positive themes and friendships.

They usually have colorful and cute illustrations that appeal to kids. The stories often focus on simple but fun adventures and lessons.

Maybe the kid ate something bad or got sick. That could be the reason for puking in the cartoon.