def _get_children(heap, idx):
    """Get the children index-value of the input index."""
    length = len(heap)
    idx_left = 2 * idx + 1
    val_left = heap._idx2val(idx_left) if idx_left < length else None
    idx_right = idx_left + 1
    val_right = heap._idx2val(idx_right) if idx_right < length else None
    return idx_left, val_left, idx_right, val_right