there are two types of dependents
1) qualifying relative. one requirement for a child to be a qualifying relative (QR) is that they are not the qualifying child of another taxpayer. you have been granted guardianship which is not legal custody
2) qualifying child - the major requirement, other than being kin, to be claimed as a QC is that any absence from the taxpayer's principal residence is temporary. Thus it seems that at some point the child can be returned to either parent making their absence temporary.
because the child under these circumstances would be a QC they can not be a QR which means you can't claim them
