How to get all children by id list?

Question

C++ / C#

I have a table with fields: Id, ParentId It is
necessary to implement multiple editing - that is, I change Id, and all ParentId changes. I wrote the following bad code:

static private IEnumerable<Guid> DeleteDepartmentsWithChildren(
    IEnumerable<Guid> departmentIds,
    DbSet<Department> departmentRepository )
{
var deletedDepartments = new List<Guid>();
foreach ( var i in departmentIds )
            {
                var departmentChildren = departmentRepository.Where( p => p.ParentDepartmentId == i ).ToList();
                foreach ( var child in departmentChildren )
                {
                    var deletedChildrenIds = DeleteDepartmentWithChildren( child.Id, currentOperatorId, departmentRepository );
                    child.MarkDeleted( currentOperatorId );
                    deletedDepartments.AddRange( deletedChildrenIds );
                }
            }
}

I don't like the fact that I'm doing a kveri at every step, this is wrong, I would like to just get all the children for all the existing parents.
These lines can probably be replaced somehow. For example, use join, but I don't know how

foreach ( var i in departmentIds )
                var departmentChildren = departmentRepository.Where( p => p.ParentDepartmentId == i ).ToList();

Answer 1

did so

var departmentChildren = departmentRepository.Where(
                dr => departmentIds.Contains( dr.ParentDepartmentId ) ).ToList();

but the problem is that it's long, so it's better to use Join
how to do it right?