Given that you've started in OnePass for everyone, I would continue to build that account for everyone as long as you can ensure that your spouse and daugther can have activity once every 18 months.

Although you live in DTW, you can still buy NW tickets and credit NW flights to your CO account. Unless you have some compelling reason to want to build in NW (e.g., you want to bank WorldPerks miles so you can redeem an award on Cebu Pacific which is partners with NW but not CO), I wouldn't orphan the CO miles and throw away the modest head start you three already have.