Even if you use miles for one ticket, get everyone a SkyMiles number. Award tickets now earn MQMs (status miles, including credit toward lifetime status) and redeemable miles don't expire, so there's no reason not to get FF account numbers for wife and kid.

BTW, are the flights you're considering operated by DL or VS? That could impact the advice you're getting about whether to purchase premium economy (DL calls it PS). Will you necessarily take a nonstop or are you looking at some domestic connecting flights (which would be in coach even if you buy PS tickets)? About two thousand dollars RT for PS sounds surprisingly cheap to me, so are you sure it's booking into PS in both directions for the TATL segments?