Apple ID or account for my kids

I do not have an iPhone so I'm not sure of how the apple ID works. 3 years ago we got my son an iPhone and I used my email as his username.

Now we just got my youngest his own iPhone and some of his brothers contacts and items are showing up on his phone. How do I go about giving my older son his own account or somehow separating the two but still using family sharing? I'm not even sure I'm asking the question right but thank you for any help!!