Z-index问题：在父容器后面堆叠子元素

I have a situation where in there are 2 children in a parent container. The first child occupies the entire content of the parent container. The other child should be below the parent container. Currently it shows up on top of the parent. I am struggling to stack the 2nd child element behind the parent container. I have a situation where in there are 2 childre