It's not uncommon for their to be coach seats avaliable for sale but no "saver" level award seats.
If you haven't already, pick up the phone and call instead of relying on the website. The website shows only NW award flights, while the phone agents can also search for award seats on CO and DL.
Failing that, also look into splitting up (you on one flight, spouse and kid on another); or into alternate dates.
Good luck.