As I said, this is hardly a scholarly work of linguistics, but the word’s inclusion in this particular primer does suggest (a) the work is unique or particular to Pennsylvania Dutch country and (b) it would be familiar to young children of that community (I believe such worksbooks are usually targeted at children around 54-5 years old).
At the outset, a caller introduces herself as a veterinarian who moved to Selinsgrove, PA (in central PA) 20 years prior to calling (so the timeperiod she spent and encounters theseencountered the words isbelow must have been between 1990 and 2010).
In the name of scholarship, however, I did a search in both the Corpus of Contemporary English (COCA) and Google nGram Viewer, and both show nixie (so spelled) is virtually unused in [written] English, particularly not contemporary [written] English, and when it is, it’s used in an unrelated sense (Nixie tubes mostly, then water sprites, then a proper noun / given name Nixie).