how do i' find a phone that has been stolen using find my phone this would be my family members phone

new to iphone my grandson had friends over they left , i think his phone left with them cant prove it . so i need to track his phone i dont know how to do that i have icloud on my phone but i need to see where his phone is does anyone know how to do that phones are only 2 weeks old so we didnt have time to set everything up on them