A muggle wife has an affair with a wizard (without knowing he was a wizard). 11 years later the family has relocated to Irvingly for whatever reason (maybe related to one of the existing muggle familys? Maybe they've been here a while?) and their 10 year old is about to receive his/her letter for hogwarts (kid's act(s) of magic have been minimal so far). Cue drams and fun times.

*bonus points if the muggle fam is very devout and uncomfortable with magic