In Harper Lee's To Kill a Mockingbird, you are referring to Chapter Twelve, when Calpurnia takes Jem and Scout to her church, First Purchase. Calpurnia has the children because Atticus has been called away for two weeks to an emergency session of the state legislature.
When they arrive, the men take off their hats in respect and the women adopt the "body language" of "respectful attention." The only person that seems unhappy about Jem and Scout's attendance is Lula.
…standing in the path behind us was a tall Negro woman. Her weight was on one leg; she rested her left elbow in the curve of her hip, pointing at us with upturned palm. She was bullet-headed with strange almond-shaped eyes, straight nose, and an Indian-bow mouth. She seemed seven feet high.
I felt Calpurnia's hand dig into my shoulder. "What you want, Lula?" she asked, in tones I had never heard her use. She spoke quietly, contemptuously...
"Don't you fret," Calpurnia whispered to me, but the roses on her hat trembled indignantly...
Jem said, "Let's go home, Cal, they don't want us here—"
I agreed: they did not want us here. I sensed, rather than saw, that we were being advanced upon...
This section alludes (hints) at the fear and discomfort the children experience with Lula's behavior because white children are attending her black church. We know the children are worried because Lula "seemed seven feet high." To a child, a grown woman with a bad attitude would elicit a fearful response. We know that the children are frightened because Calpurnia whispers words of encouragement to Scout, telling her not to worry. We know that the children both believe that Lula's threat represents the feelings of the other members of the church.
However, the people that surround Jem and Scout are there to protect and welcome them. Excuses are made for Lula's inappropriate conduct, and Zeebo (Calpurnia's son) smiles and welcomes the children.
We don't see a sentence that says the children are afraid of Lula. However, we learn from their descriptions of the event and Calpurnia's defense of them that Scout and Jem are afraid of Lula, through indirect characterization (their interaction with other characters).
Indirect characterization comes from what a character says, what a character thinks, the way other characters affect a specific character...
...How do other characters feel or behave in reaction to the character...
...what the character does, or how he looks.
When we look at the events in the story or clues that appear within the text, it is called "drawing an inference," though it is not directly stated. In other words, drawing an inference is…
...to draw a reasonable conclusion from the informaion presented.
These are the ways we learn that the children are frightened of Lula, without Harper Lee specifically saying so.
In Chapter 12 of To Kill a Mockingbird, their substitute mother, Calpurnia, takes Jem and Scout to her church on a Sunday. That she loves the Finch children as a mother is apparent by the starch that she puts into Scout’s dress and the shine that she puts onto the girl’s patent-leather shoes:
“I don’t want anybody sayin’ I don’t look after my children,” she muttered. “Mister Jem, you absolutely can’t wear that tie with that suit."
When Calpurnia and the children arrive at the First Purchase African M.E. Church, the men remove their hats and the women crossed their arms, a gesture of respectful attention. That is, all but one, Miss Lula, a “tall Negro woman.” “What you up to, Miss Cal?” she asks.
As Calpurnia’s hand digs into her shoulder, Scout hears,
“I wants to know why you bringin’ white chillun to nigger church.”
“They’s my comp’ny.”
However, when the roses on Calpurnia’s hat “trembled indignantly,” and Lula advances on Calpurnia, Jem and Scout become apprehensive. Then, after hearing Lula confront Calpurnia by saving that the children have no right to be in their church, Jem suggests to Calpurnia, “Let’s go home, Cal, they don’t want us here,” a statement which indicates his and Scout’s having been frightened by Miss Lula.
The aggressive behavior of Lula and the verbal battle of Calpurnia with her certainly indicate a tense situation, one in which the children would be frightened since they are out of their environment and Lula presents a rather formidable presence.