Do I add my child? That’s a grandchild as a dependent when I am the guardian.

You are asking whether you may claim a grandchild as a dependent.  You are the legal guardian of the grandchild.  

Perhaps.  A dependent must meet the requirements of a qualifying child or a qualifying relative in order for you to claim them on your tax return. 

The tests for a qualifying child are:

• Relationship: Must be your child, adopted child, foster child, brother or sister, or a descendant of one of these (grand or nephew).
• Residence: Must have the same residence for more than half the year.
• Age: Must be under age 19 or under 24 and a full-time student for at least 5 months. They can be any age if they are totally and permanently disabled.
• Support: The dependent must not have provided more than half of their own support during the year.
• Joint Support: The child cannot file a joint return for the year.

IRS Publication 501 page 12 defines foster child.

• A foster child is an individual who is placed with you by an authorized placement agency or by judgment, decree, or other order of any court of competent jurisdiction.

The tests for a qualifying relative are:

• Qualifying Child: They are not the “qualifying child” of another taxpayer or your “qualifying child.”
• Gross Income: The dependent being claimed earns less than $5,050 in 2024.
• Total Support: You provide more than half of the total support for the year.
• Member of Household or Relationship: The person (a friend, girlfriend, non-blood relative) must live with you all year as a member of your household or be related to you.

TurboTax software will ask you simple questions and give you the tax deductions and credits for which you are eligible based upon your answers.

See also this TurboTax Help.