Your client may be able to claim a tax credit for each child he/she have under the age of 17, if they make under $75,000 and if their filing status is single or under $110,000 if married and filing jointly or under $55,000 if married and filing separately. If your client's income exceeds the amount allowable for his/her filing status, they may still be eligible for the credit but the amount will be less. In order to claim the credit, the child must be their dependent, their natural child, adopted child, step child, foster child, grandchild, great grandchild, or great great grandchild and must be a US citizen or resident.
To compute this credit, your client must use the IRS Child Tax Credit Worksheet. Basically, this sheet will reduce their credit by $50 for each $1,000 that their modified adjusted gross income is over the amount allowable for their filing status. A child tax credit is normally not refundable if it exceeds your client's taxes.
If your client has 3 or more children and the regular child tax credit exceeds their taxes, they may be able to use the Additional Child Tax Credit to get a refund. If they claim the additional child credit, a refund is generated from Social Security and Medicare taxes that were withheld from their pay. To compute this credit, use the IRS Child Tax Credit Worksheet as you would for the regular child tax, then fill out IRS Form 8812, Additional Child Tax Credit for your client. This credit should not be used in combination with the regular child tax credit.