contract Factory {
  // ...
  function newChild() returns (address) {
    Child c = new Child();
    return c;
  }
  // ...
}

contract Child_v1 {
  // ...
  public getVersion() constant returns (uint) {
    return 1;
  }
}

contract Child_v2 {
  // ...
  public getVersion() constant returns (uint) {
    return 2;
  }
}

contract Factory {
  // ...
  function newChild() returns (address){
    Child c = new Child(); // should use bytecode as provided below
    return c;
  }
  function setChildCode(bytes[] bytecode) {
    // Some magic that updates the bytecode used in newChild() above
  }
}