This is the 9712th most frequent Spanish word.
asustar
To scare or frighten.
In this sentence, 'asustar' is used in its transitive form, indicating that the thunder caused fear in the subject (the child).
El trueno durante la tormenta asustó al niño.
The thunder during the storm scared the child.
Here, 'asustar' expresses an intent to evoke an emotion, specifically fear or concern, in the listener.
No quería asustarte con las noticias.
I didn't want to scare you with the news.
In this case, 'asustar' is used reflexively with 'me' to indicate that the action of attempting to scare was ineffectual for the subject.