From another thread, I got this solution to do a recursive selectinload:

  child_select = selectinload(Parent.child)
  for _ in range(<depth>):
    child_select = child_select.selectinload(
      child_select.nodes)

  statement = select(Parent).filter(
    Parent.id == parent_id).options(child_select)
  result = await session.execute(statement)
  parent = result.scalars().first()