The amount parents could claim jumped from $2,000 (£1,464) per child annually, to up to $3,600.
The benefits are also paid out monthly, rather than just when families file their annual taxes, meaning the poorest get help sooner.
However, you have to be registered with the Internal Revenue Service, the US tax agency, to claim the help. And up to 2.3 million families are not, according to the government.
What’s more, experts say these families are likely to be the ones most in need of help.