How can I teach Siri how to spell my child's name correctly in texts?
Siri isn't the brightest bulb in the box and she often misunderstands
  unusual words and names. Fortunately, you can teach her how to speak
  properly by adding phonetic pronunciations to your contacts.
  
  How do you do this? Watch this video or follow these
  instructions:
  
  
  Open up your uniquely-named contact and tap the Edit button up top. 
  Scroll down to the bottom of the contact page and find the area that
  says "Add New Field." 
  From your options, choose "Phonetic First
  Name" (or "Phonetic Last Name," depending on which one is the
  problem). 
  Enter in the phonetic version of the problematic name and
  tap Save. Now when you ask Siri to recognize or say a name, she'll
  get it right.
  


Source: Teach Siri to Correctly Recognize and Pronounce Names in Your Address Book