Under pre-TCJA law, the child tax credit was $1,000 per qualifying child, but it was reduced for married couples filing jointly by $50 for every $1,000 (or part of $1,000) by which their Adjusted Gross Income (AGI) exceeded $110,000. (The threshold was $55,000 for married couples filing separately and $75,000 for unmarried taxpayers.)
Starting in 2018, the TCJA doubles the child tax credit to $2,000 per qualifying child under age 17. It also allows a new $500 credit (per dependent) for any of your dependents who aren’t qualifying children under 17. There is no age limit for the $500 credit, but the tax tests for dependency must be met.
The TCJA also substantially increases the “phase-out” thresholds for the credit. Starting in 2018, the total credit amount allowed for a married couple filing jointly is reduced by $50 for every $1,000 (or part of $1,000) by which their AGI exceeds $400,000. The threshold is $200,000 for all other taxpayers. So, if you were previously prohibited from taking the credit because your AGI was too high, you may now be eligible to claim the credit.