I had a teacher in school(a man) who told us that on sunny shabbos afternoons he would go for walks and often would pass high school girls. He would say ‘good shabbos’ but the girls would completely ignore him and walk right past. He would turn around and yell ‘I said good shabbos!’ prompting the girls to sheepishly turn around and mumble good shabbos.
It is plain rude to ignore someone when they speak to you.
How would you feel if you greeted someone and they acted as if you didn’t exist? And how much more so would it hurt if that person was a relative of yours, a cousin or great aunt? If one of your Jewish brothers or sisters wishes you a good shabbos it is only polite to respond in kind.
Greet everyone(boy or girl)with a pleasant smile and friendly hello. Such a simple thing will get you very far in life and make you well liked by others.