That's true: in US you can legally marry a child ("the youngest girls to marry in 2000-2010 were three Tennessee 10-year-old girls who married men aged 24, 25, and 31, respectively, in 2001", https://en.wikipedia.org/wiki/Child_marriage_in_the_United_S...), but not your own child.

Although I'm sure there are ways around it if you are wealthy enough.

(ps "Human Rights Watch pointed out that Afghanistan has a tougher law on child marriage than parts of the United States")

Tennessee law has changed and there are now minimum ages. Surprisingly, there is no minimum age in CA; it's up to the courts.