SQLA data loss

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, Text, ForeignKey

sa_engine = create_engine("sqlite:///:memory:")
Session = sessionmaker(bind=sa_engine)
session = Session()
Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    name = Column(Text)
    children = relationship('Child', backref='parent')

    def __repr__(self):
        return self.name

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    name = Column(Text)
    parent_id = Column(Integer, ForeignKey('parent.id'))

    def __repr__(self):
        return self.name

Base.metadata.create_all(sa_engine)

p = Parent(name='Thomas')
session.add(p)

c1 = Child(name="Mary")
c2 = Child(name="John")
c3 = Child(name="Kenny")

p.children.append(c1)
p.children.append(c2)
p.children.append(c3)
session.commit()

p = session.query(Parent).get(1)
print(p.children) #prints [Mary, John, Kenny]

temp1 = p.children[1]
temp2 = p.children[2]

p.children[2] = temp1
p.children[1] = temp2

print(p.children) #prints [Mary, Kenny, John]

session.commit()
print(p.children) #prints [Mary, John]. Oh my God! They killed Kenny!