Christine's subplot was originally written and filmed for "Guarding Greg." The A-plot in that episode ran too long and the school scenes had to be nixed. Rather than toss them completely, "The Good Dad" (with a shorter A-plot and by the same writers) was tweaked to fit them in.
Guest star John Ducey had previously tried out for the role of Greg. Check out his website for some behind-the-scenes info on the making of this episode.