To be eligible for the child tax credit you must have at least $2,500 of earned income from work. The SS benefits received are not earned income.
If the Social Security benefits are the Only income to be reported on the tax return, then there is no reason or need to file a tax return.
Unless you also received a Form 1095-A for your health insurance through one of the State Marketplace Exchanges or from healthcare.gov then you must file a tax return.