How do you create a ip pin for A child

You don’t need a special number. If the child lives with you tax says only you can claim the child unless you give him written permission to do so.  If he claims the child the IRS will sort it out and assess him penalties, interest and additional taxes. 

See this  from the IRS

https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin

Using the IPPIN you get the child even if you are the last to file.