What is the most fuel efficient hatchback/wagon that will seat 5 (3 of those 5 are in carseats!)?

Hatchback go for a 4 door Golf TDi.

Consider leaving someone behind – 4 passengers are the most you'll get out of standard sedans, wagons and hatchbacks. Go any higher and you won't see much of a fuel-efficiency benefit beyond the most fuel-efficient minivans, which are the 4-cylinder Dodge Caravan at 20/26 city/highway mpg and the 6-cylinder VCM Honda Odyssey at 20/28. Leaving one person behind will save you 10 mpg or more – the Honda Civic gets 30/40, the Toyota Corolla gets 30/38 and the Honda Fit or Scion xA gets 31/38. If all else fails, consider getting a Toyota Highlander hybrid. It has available third-row seating and gets 33 city, 28 highway.

toyota matrix , pontiac vibe, scion xB,xA,honda fit, or the 2007 nissan versa which they are on sale in some nissan dealers now it will come at tour local nissan dealer within two weeks to next month if they are not there. there is no 3rd row seats on any hatchback and that is for sure

I think you should think of a mini Van rather than a hatch back/ wagon