The ExpandContainer supports a single child. You can place any control inside the container. This can be a StackPanel, Grid, etc.
Expandable containers are also supported by the LayoutControl, which is a control providing advanced layout features. To create an expandable container within a LayoutControl, create a LayoutGroup and set its LayoutGroup.AllowExpand property to true.
The following example creates an ExpandContainer containing a Grid with sample controls: