Saving to the gallery is optional.
I was able to add a child sim to any household without first saving them to gallery. You don't need to start a new game either, you can pull up any old game choose any household you want to play with genetics - enter manage household CAS screen and add.
If you don't like who you added, you can always go back to manage household CAS and remove the child - send him/her to another household and you will have your old household back. I know that's mean, sending a child away.