trouble understanding how to query related tables with SQLAlchemy

I have created the following database definitions and test data:

from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm.session import sessionmaker
Base = declarative_base()
# build dynamic table
cols = {"__tablename__": "mytable", "A": Column(String, primary_key=True), "B": Column(Integer)}
table = type("mytable", (Base,), cols)
cols2 = {"__tablename__": "mychild", "A": Column(String, primary_key=True),"Z": Column(Integer)}
cols2['parent_id'] = Column(String, ForeignKey('mytable.A'))
child_table = type("mychild", (Base,), cols2)
child_table.parent = relationship('mytable', backref='mytable', primaryjoin='mytable.A==mychild.A')
# initialize engine
from sqlalchemy import create_engine
engine = create_engine("sqlite:///", echo=True)
Base.metadata.create_all(engine)
# generate a toy dataframe and dump to sql
import pandas as pd
df = pd.DataFrame.from_dict({"B": {"row1": 1}})
df.to_sql("mytable", engine, if_exists='append', index_label="A")
Session = sessionmaker(bind=engine)
session = Session()
session.query(child_table)

And this leads to the error

ArgumentError: Could not locate any relevant foreign key columns for primary join condition 'mytable."A" = mychild."A"' on relationship mychild.parent.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or are annotated in the join condition with the foreign() annotation.

In general, I am confused on how I can write queries against my tables. In this case, how would I do something like select * from mytable or even more useful SELECT B,Z from mytable JOIN mychild using A, which is what I am really trying to achieve through this.