Cosmic King:
Maybe they got their mind manipulated (it worked on Oh My Goddess)
cata:
Interested question. Let me think... Lum was in a school trip (like Kintaro) and she liked so much of Earth that she decided to stay.
Kroptik:
I don't see her parents allowing her... But she could go on holidays to Ataru's house every summer...
cata:
Well they let Ten live with Lum and Ataru.
Kroptik:
It's different since Lum is his cousin.
Do you think your parents would let you live at a friend's house in another country when you were 5 years old?? Of course not. Parents want to watch over their children. It's only natural.