A child's earned income (for example, Form W-2) should not be reported on the parent or guardian's tax return. Whether a dependent has to file a return generally depends on the amount of the dependent’s earned and unearned income and whether the dependent is married, is age 65 or older, or is blind. If required, the child will need to file a tax return. Keep in mind that if either federal or state taxes were withheld from the pay, only by filing a return can the child receive a refund for any amounts due back to him or her (even if not required to file).
Per IRS Publication 929 Tax Rules for Children and Dependents:
Page 3 starts with the Filing Requirements for children and dependents. You may also wish to use Table 1 on page 4 to help determine if your child or dependent needs to file a tax return in their situation.
Page 6: Responsibility for Child's Return
Generally, a child is responsible for filing his or her own tax return and for paying any tax, penalties, or interest on that return. If a child can’t file his or her own return for any reason, such as age, the child's parent, guardian, or other legally responsible person must file it for the child.
Signing the child's return. If the child can’t sign his or her return, a parent or guardian must sign the child's name followed by the words “By (signature), parent (or guardian) for minor child.”
Authority of parent or guardian. A parent or guardian who signs a return on a child's behalf can deal with the IRS on all matters connected with the return.
In general, a parent or guardian who doesn’t sign the child's return can only provide information concerning the child's return and pay the child's tax. That parent or guardian isn’t entitled to receive information from the IRS or legally bind the child to a tax liability arising from the return.
Third party designee. A child's parent or guardian who doesn’t sign the child's return may be authorized, as a third party designee, to discuss the processing of the return with the IRS as well as provide information concerning the return. The child or the person signing the return on the child's behalf must check the “Yes” box in the “Third Party Designee” area of the return and name the parent or guardian as the designee.
If designated, a parent or guardian can respond to certain IRS notices and receive information about the processing of the return and the status of a refund or payment. This designation doesn’t authorize the parent or guardian to receive any refund check, bind the child to any tax liability, or otherwise represent the child before the IRS. See the return instructions for more information.
You may also wish to utilize the IRS Interactive Tax Assistant (ITA) topic Do I need to file a tax return?