Making a tree structure in django models?

I want to have a model with 2 fields, children and parent. How do I do this in django? I have something like this

from django.db import models
class FooModel(models.Model)
    parent = models.ForeignKey('self', blank=True, null=True)
    children = models.ManyToOneRel('self', blank=True, null=True)

    def __init__(self, *args, **kwargs):
        super(FooModel, self).__init__(*args, **kwargs)
        self.parent.children.add(self)

But I don't think i'm supposed to use the ManyToOneRel like this (especially because it's giving me a keyword error on 'blank'). Any advice?