Maybe its a California thing but I don't know one single kid, and trust me I am around a lot of kids, anyway I don't know one kid that calls anyone mr or mrs so and so, unless of course it is a teacher, that is for sure mr or mrs/ms.

I think I would start out with the mr or mrs with the in laws and as the relationship progresses decide between all of you what feels best. Of course it may be something we can't type here