I don't want to do a formula, because I want to give the users the ability to edit if needed.
I don't want to do a workflow, because I don't want the users to have to enter this information as it already on the Opp and if we use a workflow it won't populate until Save.
2 answers
Are you using lightning ? If yes then you can use global record level actions to pre populate the Information from parent to child. You can set there pre defined values using formula editor.
