The very best place in this entire area to go sledding is in Ford Field, in Dearborn. It is the largest piece of land perfect for sledding in at least a 20 mile radius. Seriously.
Dearborn's Ford Field Park can be accessed off of Brady Street, north of Michigan Avenue in west Dearborn. If you park and walk through the bridge- look straight ahead and the vast open space and gigantic hills laying before you.Check out the map on this website. You can access the hills directly by driving up Cherry Hill, I believe.
Map:
http://events.detnews.com/dearborn-mi/venues/show/64704-d...
I've been taking my 4 year old there every year since he was one. There are 90 degree hills for older kids and smaller ones for the little guys. It is perfect for the whole family.