I support my 2 grandchildren as a retired on not taxable soc security

[xmasbaby0]

Even without taxable income if the grandchildren are your dependents you may get the child tax credit and the 3rd stimulus by filing a 2021 tax return.  Are the children under the age of 18?   Claim them as dependents on your 2021 tax return.   The child tax credit will show up on line 28.   If you did not yet receive the stimulus checks for claiming them you can use the recovery rebate credit in Federal Review to get the $1400 per person amount and it will show up on line 30.

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2021 taxes as long as they meet the following requirements:

Qualifying child

• They're related to you.
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They're under the age of 19 (or 24 for full-time students).
• No age limit for permanently and totally disabled children.
• They lived with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year (exceptions apply).
• They made less than $4,300 in 2021.
• You provided more than half of their financial support.

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them. There may be other tax benefits you can get when you claim a dependent.

[SweetieJean]

Do you have any other income of any type?  Do the children's parents live with you?

[Hal_Al]

There are two more rules for a "Qualifying Child" (QC) dependent:

1. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child (this essentially means that you have the parent’s permission to claim the child, if the child also lived with the parent more than half the year)
2. If the parents of a child can claim the child as a qualifying child but no parent so claims the child, no one else can claim the child as a qualifying child unless that person's adjusted gross income (AGI) is higher than the highest AGI of any of the child's parents who can claim the child.

Note the difference in the support test rule for a QC and a QR. If the children don't live with you, it's unlikely that you provide more than half their support, if your only income is non taxable SS.

The money you hear about people getting for just filing a tax return claiming kids requires them to  have some earned income (wages or self employment). Without earned income, they are not eligible for the "refundable" Earned Income Credit or Additional Child Tax Credit.  Both credits are calculated on the amount of earned income you have. No earned income means no tax benefit. A small amount of earned income means a small refund. The child tax credit does not "kick in" unless you have at least $2500 of earned income.  That earned income  rule was not fully true for 2021, because of the Covid rules. Stimulus/recovery rebate money was available for kids and the earned income rule for the (additional) child tax credit was suspended for 2021 (but not 2020).