Actually 7 歲既男仔都可以好鬼naughty. And the incident occurred because your son has broken the MP3 of your daughter !!
I think we should not only blame your daughter lor. Maybe your son didn't apologize for what he did and your daughter didn't know what to do at that time and could only think of sth she got from her daily life (in particular, TV) and ask him to knee lor.
I suggest you could pretend to check/borrow the MP3 from your daughter and then investigate what she thinks about the broken MP3 first. Don't start of by scolding.
Please do not quote any information posted by fungwongphie. 不誠實使用電腦會構成罪行,切勿以身試法。 何謂不誠實使用電腦??