Scene 5 is pivotal towards the uncovering of the real Blanche Dubois. Having presented herself at her sister's house wearing her best garments and acting in her utmost polite ways, Blanche is actually a social runaway who has run out of funds, and whose reputation precedes her. For this reason, it is imperative that Blanche conceals every single truth about herself, including the name of her former acquaintances.
To her disgrace, Stanley-of all people-finds out about a man named "Shaw", who presumably knew Blanche quite well. This information fills Blanche with terror. As a result, she accepts a soda from her sister, and laces it with alcohol to calm herself down.
When Stella leaves to meet Stanley at the bar, Blanche stays alone waiting for Mitch. When a young man comes to collect money for The Evening Star newspaper, Blanche is already drunk and uninhibited. She first begins to flirt with him by hitting his ego and by likening him to a "prince" from Arabian Nights. That alone is quite a dangerous comment to make to a child, since just by reading that type of literature alone denotes Blanche's secret, perverse desires.
Blanche then kisses him following her actions with the words,
Now run along, now, quickly! It would be nice to keep you, but I've got to be good - and keep my hands off children.
This is indicative of several things things: first, that Blanche breaks down completely under difficulty, that she depends on alcohol to suppress her emotions, and that alcohol makes her uninhibited with men. Remember that alcohol was also involved the night of the suicide of her husband.
Second, it shows that this is a pattern of behavior. Blanche had already been fired from teaching in a secondary school. Judging from her behavior, it seems that she does have a perversion for younger men, even if they are still adolescents.
Third, it hints at Blanche's mental chaos. She may not actually be in tandem with reality. She may just be a sick woman, for all we know. However, she is not too sick not to know the consequences of her actions; she knows all too well what would happen if she had continued her advances with that boy. Hence, between the lose nature of her behavior, her penchant for indiscretion, and the possibility that she hides a kind of sadistic or perverse streak are all reasons why Blanche kissed this unassuming boy.