Why was the mother angry with little Francis in The Swiss Family Robinson?

In The Swiss Family Robinson, the family is marooned on an island. They are very fortunate to land on the island because it provides them with a lot of living animals and food. Unlike other desert island narratives, this story follows the successful colonization of an uninhabited island.
The family has six members: William, Elizabeth, and their four sons. Franz, or Francis, is the youngest of the children. His place as the youngest child also makes him the most closely attached to Elizabeth, who is continuously looking out for his well-being and happiness. The older boys tend to gravitate towards their father, but Francis is always helping his mother.
In chapter 3 of the novel, Francis is helping his mother start a fire by gathering sticks. He goes off by himself to find more sticks for their fire, and when he returns, he is chewing on something he found on the way. The story says,

Just then little Francis came up with a large bundle of sticks, and his mouth full of something he was eating with evident satisfaction.
"Oh, mother!" cried he, "this is so good! So delicious!"
"Greedy little boy!" exclaimed she in a fright. "What have you got there? Don't swallow it, whatever you do. Very likely it is poisonous! Spit it all out this minute!" And his anxious mother quickly extracted from the rosy little mouth the remains of a small fig.

Elizabeth is distraught because she is frightened by the possibility of Francis being poisoned by eating something he finds in the wilds of the island. She tells him that he is “greedy” because at its heart, the story is a morality tale, and the parents often act as paragons of virtue that correct their wayward children.
It ends up that Francis finds a grove of figs that the family can eat. The anger his mother feels is derived from love and concern, which fits with how she treats him in the story, but it is interesting because the parents rarely get visibly angry with their children in the story.