In Germany, the informal form of "you" is "du." The person that you can appropriately address with the informal "du" is your ten-year-old cousin. It is common to use the informal "du" with family members and close friends, especially those who are younger than you. Therefore, when speaking with your ten-year-old cousin, you can use the informal "du" to address them.
In Germany, it is appropriate to address your ten-year-old cousin and your roommate's father with the informal "du" (you). However, it would not be appropriate to address a university professor or your new supervisor at work with the informal "du". In those cases, it is more appropriate to use the formal "Sie" (you).
