I have only one child, and she's 17 now. Basically, you take your cues from the kid on how they want to play (up to a point). She wanted me to make up stories

I have only one child, and she's 17 now. Basically, you take your cues from the kid on how they want to play (up to a point). She wanted me to make up stories for her when she was little, so I did that. She also would play "coffeeshop" or "train", where she ran things and my wife and I were the customers. Basically, some kids really want the adults to play, some aren't as into that, and that's ok. So I think it's entirely possible that your niece and nephew look for something different from their uncle/aunt than from their mom/dad. They'll let you know.