You can select the children of a member and other members within the Hierarchy Editor.

Next to each selected member is a dropdown icon.

SelectionDescription
ChildrenAll direct children of the selected member will be inserted into the report.
DescendantsAll descendants will be inserted.
Lowest DescendantsInsert the lowest-level descendants of the selected member.
Descendants atChoose to display the descendants of the selected member at a particular level. For example, the descendants of Bikes at the Product level.
Next/PreviousThis'll select the current member and the next/previous members in the same level. This is particularly useful for date selections, such as the current month and previous 11 members. This value can also be driven from an Excel range.