If the parents are not filing then the child will file their own return.
They will need information from the parents to file their return, even if the parent(s)
do not file.
If the parents will not supply that information, then you'll have to reach out to the IRS to get this information.
According to Parent's return information not available in the IRS's 2020 Publication 929, Tax Rules for Children and Dependents (draft),
If a child can’t get the required information about his or her parent's tax return, the child (or the child's legal representative) can request the necessary information from the Internal Revenue Service (IRS).
A procedure in the document describes the steps you should take to request this information.
