Yes, you can (often) use often + simply past. :-)
Often is an adverb in this context.
Using often adds information. With the examples,
When I was young, I went to my dad's office to help him in his work.
says that you (the speaker) went to your dad's office, but gives no indication whether it was a single time or repeated. The inclusion of often provides that information.
The other case,
When I was young, I would go to my dad's office to help him in his work.
implies that it was a repeated event, and the inclusion of often adds information about the frequency of the event.
Other examples with simple past are:
I often ate quickly, but now I eat more slowly.
I was often late for school.
Sometimes it helps to consider whether you can use other frequency or comparative specifications in the sentence. If you can use words such as seldom, regularly, frequently, occasionally, usually, etc, then you can also use often.