Social Security Number lookup.

[piedratammy10]

Look up dependant"s Social Security Number

[xmasbaby0]

If you have claimed the dependent on a tax return in a past year, you can look on an old tax return.  

  You have to access your own account and/or  print it for yourself using exactly the same account and user ID that you used when you prepared the return.

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/270...

Many people have multiple TT accounts and forget how to access them.  Log out of the account you are in now.

Account recovery

https://myturbotax.intuit.com/account-recovery/

https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password

The child's school or the doctor you go to *might* have a record of the SSN.    

Or you will have to get it from Social Security.

SOCIAL SECURITY LINKS

https://www.ssa.gov/myaccount/replacement-card.html

To Replace a Child’s Social Security card

https://faq.ssa.gov/en-US/Topic/article/KA-02723