You'll have to create the child part just like you would create a normal WTPart . Then using WTPartUsageLink you need to link the parent and the newly created WTPart as child.
From object | From relation | Link object | To relation | To object | Comment |
wt.part.WTPart | usedBy | wt.part.WTPartUsageLink | uses | wt.part.WTPartMaster |
|
Windchill Java doc will have further info on this.
-Malavika