My brother son ask me about seXx I wonder how to explain to this small kid

Well if it were my child then I would just tell the truth and keep ti as simple as possibe . How much detailI give would be depend on the age. As for telling stuff to someone elses child...NO!!

A simple explanation to the kid will do just fine. No need to tell him the details. Coz if you do I bet he will ask more and it will be more difficult for you to explain.

PLEASE READ! I have a 12 year old brother, we live with only my mom and me being the man of the house, I had to talk about it with him. Don't talk about only seks!!! Talk about all of life, how he has to be a determined person, how he has to study and get a good job, how he has always to think before doing something, to not let his emotions make his life worse. Than get into the matter of seks explaining WHY two people get together, how it usually happens and the possibility's. Talk about protection, emphasize the matter of THINKING before doing something (so he won't let his desires make him regret his decisions)

You can educate small kid about it. Most kid eager to learn about that stuff coz their opposite also assess their growth development. You need to discuss it carefully with some restriction/limitation according to age group.