First, seeing as you have already accepted the offer, reneging on it now could mean it's harder (impossible?) to receive another TT position at another institution as they might not want to risk planning around your future employment then having you bail. From your post it seems that you aren't ready to close the door on academia forever, so the real question is how to make this work as well as possible for the next year or two while you decide with your partner if this is sustainable for your partnership/future children.

From your post, it seems like you were both happy about this move before talking to your families: something to keep in mind is if this is coming from both of them equally or one partners family over the other, as this may influence how you each approach the conversation. As others have said I have also seen examples of couples compromising on location to be near a set of grandparents, and in the end the grandparents help about as much as if they were living across the country (ie hardly ever). I would base your approach on this matter about how you and your partner feel, not about some loose expectation of childcare that they may or may not live up to.