Number of Children in the United States

In 2021 there were an estimated 73.6 million people in the US aged 0-17, representing 22.2% of the total population.

According to the Federal Interagency Forum on Child and Family Statistics, in 2021, of all US children:
49.4% were White, non-Hispanic
13.8% were Black, non-Hispanic
0.8% were American Indian or Alaska Native, non-Hispanic
5.4% were Asian, non-Hispanic
0.2% were Native Hawaiian or Other Pacific Islander, non-Hispanic
4.7% were two or more races, non-Hispanic
25.7% were Hispanic

Source: 

Federal Interagency Forum on Child and Family Statistics. (2022). America’s children in brief: Key national indicators of well-being, 2022. Washington, DC: U.S. Government Printing Office.