Final answer:
The custodial parent can fill out Form 8332 to release their claim to the child's exemption to the non-custodial parent.
Step-by-step explanation:
If the custodial parent agrees, the non-custodial parent can claim the child on their taxes. In order to do so, the custodial parent must fill out a written form called Form 8332, also known as the Release/Revocation of Release of Claim to Exemption for Child by Custodial Parent. This form allows the custodial parent to release their claim to the child's exemption and allows the non-custodial parent to claim the child instead.