Finding all child nodes in hierarchial data

The C# code that uses Linq find all child nodes of one node in hierarchical data.

public IEnumerable FindChilds(List items, int toplevelid)
{
    List inner = new List();
    foreach (var t in items.Where(item => item.ParentId == toplevelid))
    {
        inner.Add(t);
        inner = inner.Union(FindChilds(items, t.Id)).ToList();
    }
    return inner;
}