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You don't need her to sign a form releasing dependent exemptions if the children lived with you. The form for releasing dependent exemptions is to resolve issues between divorced or separated parents only. 

If your daughter lived with you as well, review the “tie breaker rules,” which are found in IRS Publication 501 if she had income. If she didn't have income, as @xmasbaby0 pointed out, you may be able to claim her as a dependent as well. These rules establish income, parentage, and residency requirements for claiming a child:

To determine which person can treat the child as a qualifying child to claim these five tax benefits, the following tiebreaker rules apply. For purposes of these tiebreaker rules, the term “parent” means a biological or adoptive parent of an individual. It does not include a stepparent or foster parent unless that person has adopted the individual.

• If only one of the persons is the child's parent, the child is treated as the qualifying child of the parent.
• If the parents file a joint return together and can claim the child as a qualifying child, the child is treated as the qualifying child of the parents.
• If the parents don't file a joint return together but both parents claim the child as a qualifying child, the IRS will treat the child as the qualifying child of the parent with whom the child lived for the longer period of time during the year. If the child lived with each parent for the same amount of time, the IRS will treat the child as the qualifying child of the parent who had the higher AGI for the year.
• If no parent can claim the child as a qualifying child, the child is treated as the qualifying child of the person who had the highest AGI for the year.
• If a parent can claim the child as a qualifying child but no parent does so claim the child, the child is treated as the qualifying child of the person who had the highest AGI for the year, but only if that person's AGI is higher than the highest AGI of any of the child's parents who can claim the child.

Subject to these tiebreaker rules, you and the other person may be able to choose which of you claims the child as a qualifying child.

See also this TurboTax tips article for more information.

@mommyrach5