In Japan, it's considered polite to use honorific language when interacting with children, especially those who are younger or in positions of authority. Instead of saying 'good afternoon', you might say '' (ohayou gozaimasu) to show respect and courtesy.
Alternatively, a more casual greeting could be '' (konnichiwa), which is the standard way to greet someone during the day. If you want to use an English phrase, you could try saying 'good afternoon' in a gentle or soft tone to indicate that it's not your native language.
When interacting with children in Japan, it's also worth noting that they may use more formal language when speaking with adults, especially if they're in school. So, using a polite and respectful greeting can go a long way in showing cultural sensitivity.