If she's serious about this then what she should do is promote a massive program of female education and equal rights in Africa, India and the Middle East. It is the one proven way to reduce population growth and is entirely voluntary and therefore humane, by virtue of the fact it gives women in these countries the CHOICE of something other than exhausting themselves having five or more children.
I suspect that will not be the focus of her efforts, however. And as regards using "children will have hard lives" as a motivating argument, that's really a testament to how terrified of hardship people are in the modern West than it is a valid argument. How hard did most of our grandparents have life? Our great-grandparents even more so. Should they not have lived and been born? And thus none of us here?
I agree the world's population should be lower. But I doubt she'll approach it in a logically consistent and equitable way. Or acknowledge the consequences of a unilateral approach to this.