OH! another thought (I'm so terrible!)
Especially as pre-teens and teens-they aren't likely to "fall for you" fast.
Something that tends to be UNMENTIONED BY THEM but important none the less-is knowing that they need to KNOW (not hear-see) that you have no intention of trying to come in as their parent. Leave the parenting duties to their parent.
If you treat them as "short adults" whose first language is NOT the same as yours-when conversing, that will help too. Remind yourself-that children and especially teens-have their own language. It sounds JUST LIKE ENGLISH-but it isn't the same as adult language. So feel free to ask what they mean by something, show a sincere interest in what they are doing (even if only to learn from it-because it's different than what you already know) and don't offer a lot of info to them without them asking.
Answer what they ask honestly and directly, but don't go into long drawn out details-because they will tune you out.