Stefan,
WTPart-to-WTPart Parent-Child links that build a WTPart BOM are stored in the WTPARTUSAGELINK table.
WTPart-to-EPMDocument owner links are stored in the EPMBUILDRULE table.
There are more tables needed to query if you're looking to pull EVERYTHING related to a specific WTPart or EPMDocument. Let me know if there's any way I can help you with an extraction.