Why was Kirsti sulking in Number the Stars?

Kirsti was sulking because Annemarie and Ellen were racing and left her behind. 

Kirsti is Annemarie’s younger sister.  She felt left out when Annemarie and Ellen decided to race each other down the street.  Kirsti was sulking because she could not keep up. 

"Go!" shouted Annemarie, and the two girls were off, racing along the residential sidewalk. Annemarie's silvery blond hair flew behind her, and Ellen's dark pigtails bounced against her shoulders.

"Wait for me!" wailed little Kirsti, left behind, but the two older girls weren't listening. (Ch. 1) 

Ellen and Annemarie have bigger problems than Kirsti.  When they are running, they get stopped by a German soldier.  He finds their running suspicious, even though that is what little girls do.  

The girls hear the soldier shout "Halte!"  This is the German word for stop, which the girls can understand. 

Behind her, Ellen also slowed and stopped. Far back, little Kirsti was plodding along, her face in a pout because the girls hadn't waited for her. 

Annemarie stared up. There were two of them. That meant two helmets, two sets of cold eyes glaring at her, and four tall shiny boots planted firmly on the sidewalk, blocking her path to home. (Ch. 1) 

The soldiers question them, and seem to think Kirsti’s sulking is cute.  One of the soldiers says that she reminds him of his own child.  They tell the girls not to run and let them go. 

The incident demonstrates the everyday ways that the German occupation affects the people of Denmark.  Two little girls cannot even run down the street, which is a common thing for little girls to do.  The soldiers get suspicious.  They are really just hassling the children.  In a way, Kirsti’s bad attitude helped.  It reinforced to the soldiers that they were just kids, and they thought her attitude was cute.