The parent that will receive the child.
The new child to attach to the parent.
Where in parent's children the child goes; out of range clamps to the nearest end, so 0 is always the front and a large index is always the back (matching Array.prototype.splice).
Insert a child into a parent's children list at a given position (like appendChild, but positioned). Appending is insertChild at the end:
appendChild(parent, child)isinsertChild(parent, child, parent.children.length).