"When I was little our family often went to France."
Translation:Когда я была маленькая, наша семья часто ездила во Францию.
Both Nominative and Instrumental are used after «был»/«была» in the past and future tenses. I personally tend to use Instumental everywhere, but both are possible.
Rosenthal's style guide (§ 177.3) recommend using Instumental for changeable things:
- она́ была́ ма́ленькой 'she was small' (but she's expected to grow up),
- она́ была́ певи́цей 'she was a signer' (but she might decide to do something else later),
and Nominative for permanent things:
- она́ была́ китая́нка 'she was Chinese' (and this fact doesn't change with time)
However, this is a prescriptive rule that doesn’t reflect how we actually speak. This sentence on Duolingo actually breaks this rule. :D
That's how I would say it. «Когда я была маленькой, наша семья...».
Note that comma is required because it's a complex sentence, that is, sentence-in-a-sentence ('I was small' is a sub-sentence, 'our family went' is main sentence). Russian generally requires subsentences to be separated from main sentences by punctuation.