Most income is considered taxable income. That being said, social security benefits for children are taxable but usually not taxed because there isn't enough other income to create a tax liability. For example, a child filing as single can have benefits up to $25,000 before any type of tax calculation takes effect.
This article from the IRS gives a good summary https://www.irs.gov/faqs/social-security-income/survivors-benefits/survivors-benefits
