In the case of child tax credit, an IRA contribution would lower the MAGI.
Here is how MAGI affects the credit:
Child tax credit may be phased out for taxpayers whose modified adjusted gross income is above the limit for your filing status.
The limits are:
$110,000 for married filing jointly
$75,000 for single or head of household
$55,000 for married filing separately
The credit is reduced $50 for every $1000 of income (or part thereof) over the limit.
