“Perhaps she can learn the hard way why she should “be prepared”.”
In general and in other circumstances, I would agree with that approach and it is one I’ve utilized myself with my own kids. However, in a group camping situation, it could result in circumstances that could affect the group or endanger the child. Being cold and wet, in nature where temperature swings are possible and where many 8 year olds don’t have much body fat, could result in an unsafe situation. Why risk it?
You either bring the proper gear on the packing list or you stay home.