The help that you want to review is the one for making relationships.
Here are the steps.
In fact, what you think of as your child record is actually the parent. I presume that the code is unique in the code table. If you already have a relationship where your data entry form record is the Parent, then go back and delete that relationship as its backwards.
Then, make the Key field in your Codes table the Code field.
Now - you make your relationship where One Code record is related to Many data entry records. It sounds like you want to have the user select a code from the drop down list so you can allow the relationship to create a field called related Code (which you can late rename if you like. Then bring in the description to your data entry record by adding the description as as look up field.
Having said all that, it might be helpful if you describe the bigger picture as to what you are trying to accomplish here, as we may be able to help more if we understand what you are trying to do. ie what is your use case?
