Parents have a responsibility to bring up their children as well as they can, and therefore, they have the responsibility to make decisions about what is appropriate for their sons and daughters. Add to that the strong emotional attachment between parents and children, you will not get a favourable response if you start to question their judgement. I have never reacted favourably to this, although it has only happened a few times. You may believe, probably correctly, that you are both older and wiser than your neice, but I would advise not making judgements unless the safety of the child is at stake. You could try questions: "Would so-and-so enjoy such-and-such," but even this is risky.