Is the Child Tax Credit that has been changed to be refundable this year up to 1400 capped at 15% of income after $4500.00. per family or per child?

If someone files Jointly with 4 kids with income of $13,833 will they get 1400 in refund for each child or only once since it has reached the 15% cap? is the 15% calculated for each child or per filler?