Your daughter wants to feel "grown up" like her cousins, so you need to in some way acknowledge this feeling and at the same time stick to your convictions. Tell her that you understand how difficult it feels for her to be the youngest and that it must be frustrating for her not to be able to do what she wants. Try and reflect her experience using a tone in your voice to match her emotions. For example, you may say something like, "It sucks to be the youngest and you just don't understand why you can't wear high heels. There just seems to be no good reason!" When children feel heard, they will usually let you know and will also be much more receptive to further conversation. Silence, at this point is always a good idea to let your acknowledgement of their emotions and experience "sink in." At this point you can say something like "I do understand what you are experiencing, however I need to do what I think is right. I need to be a good parent, so I'm going to say no to the high heels until you're a bit older." She may still be upset but I suspect not as much. You can repeat this process as many times as necessary. When she realizes that the "rule" is remaining in place, no matter what, she will stop testing the boundaries. Good luck! - Marsha

I am from another generation!  Perhaps the ability to make "decisions" should be based on how mature the child is.  Afterall, if she is still prone to having tantrums, is it realistic to think she is going to understand mature reasoning?  I believe "No" means "No" and if efforts to "reason" with her end up in tantrums... then no is still no.  It amazes me how parents try to reason with kids who are just not going to get it.

It sometimes feels counter-intuitive to acknowledge your child's feelings, particularly when they are throwing tantrums. It's frustrating and you're angry too! Try it. You'll be surprised how practicing "emotion coaching" actually takes the power struggle out of the equation. You can then practice "no means no," but now your child has a better chance of listening to you. -  Marsha