def get_children_treepos(self, treepos):
        """Given a treeposition, return the treepositions of its children."""
        children_treepos = []
        for i, child in enumerate(self.dgtree[treepos]):
            if isinstance(child, nltk.Tree):
                children_treepos.append(child.treeposition())
            elif is_leaf(child):
                # we can't call .treeposition() on a leaf node
                treepos_list = list(treepos)
                treepos_list.append(i)
                leaf_treepos = tuple(treepos_list)
                children_treepos.append(leaf_treepos)
        return children_treepos
Given a treeposition, return the treepositions of its children.