Determine the amount of the child tax credit in each of the following cases:

A single parent with modified AGI of $$213,400$ and one child age $4.$

A single parent with modified AGI of $$78,000$ and three children ages $7,9,$ and $12.$

A married couple, filing jointly, with modified AGI of $$407,933$ and two children age $14$ and $16.$

A married couple, filing jointly, with modified AGI of $$132,055$ and one child, age $13.$