Are you really talking about the child tax credit? That is money that goes to a parent or someone else who claims a qualifying child dependent. There are certain income limits and other rules. If your parents qualified and didn't claim the credit, they must amend their returns. If they didn't qualify, they didn't qualify. There is nothing for you to claim now.
Are you talking about the COVID stimulus payments? Those also went to the parent or other qualifying taxpayer who claimed a dependent. The money was for the taxpayer, not the dependent, even though it was based partly on the number of dependents a taxpayer claimed. If you were or could have been claimed as a dependent in 2020 and 2021, you were never eligible for the stimulus. Your parents either were or were not, but they have to take that up with the IRS themselves, there is nothing for you to claim now.
Are you thinking of something completely different? We would need to know what you are thinking about, but it is almost a sure thing that you can't get money now in place of money your parents didn't get in the past. If your parents were eligible for anything, they would have to claim it on their tax returns.
