How to Actually Be a Cool Adult for a Kid:
Be a constant part of their life. Show up for birthdays and holidays. Babysit. Go on outings with the family.
Listen to them starting when they're young. Show an interest in what they have to say, even if it's incomprehensible toddler ramblings, or a ten minute monologue about Paw Patrol. Let them show you their favorite toys. Get excited if they show you art or crafts, and if they give you one, treat it like a museum piece.
Treat them like a person. Respect their preferences and bodily autonomy (no demanding hugs). Don't make fun of them or embarrass them. Engage in actual conversation.
Set respectful boundaries, but give them room to explore.
Be willing to intervene if another adult is making their life unnecessarily difficult or if other kids are bothering them.
Respect the rules and boundaries their parents have set. Yeah, it's easy to win kudos with kids by letting them have fun that their parents won't, and yeah, the fact that this isn't actually your kid means you can be more relaxed in some areas, and have different rules at your house than at home. But most of the time, you will be supporting your siblings in raising their kid, so like. Be respectful of that too.
Don't be a snitch if you don't have to. If you catch the kids playing a forbidden game, or teasing one of the niblings at Thanksgiving dinner, you don't need to make it a big production. Be the adult and handle the situation, then let it go.
Keep confidence. Goes along with the previous one, but for older kids. If a kid or teen comes to you with a concern or looking for advice, respect their privacy. Don't tell your sibling about it unless the kid gives you permission to.
Accept the fact that teenagers are contrary little shits who usually think grown ups are the most embarrassing entities in existence. They may not want to hang out with you. They may find it embarrassing if you share their interests.
Remember that even if you end up being the kid's friend, you are still also one of their adults. You have responsibilities to a kid if you want to be a big part of their life, in a way you wouldn't automatically have with another adult.
Don't try to be the cool adult if you are doing it for your own gratification. Be the kid's friend because you love them and care about their well-being.
