Based on what I can see, it should be:
The columnName should be the column you want to filter. (I just removed the word child from the docs, which shouldn't have been there.)
Try adding debug: true to see if the function complains about something.
M.
$().SPServices.SPFilterDropdown({
relationshipList: "WorkPhase",
relationshipListColumn: "Title",
relationshipListSortColumn: "ID",
columnName: "Work Phase"
}); Try adding debug: true to see if the function complains about something.
M.