If you are filing as Single or Married filing Separately as soon as you enter a qualifying child it will automatically switch you to Head of Household.
Go to My Info and look at filing status.
Here's a FAQ for Head of Household
https://ttlc.intuit.com/community/family/help/do-i-qualify-for-head-of-household/00/25539
Who is a qualifying person for Head of Household
https://ttlc.intuit.com/community/family/help/what-is-a-qualifying-person-for-head-of-household/00/2...
