Vera Hut? Mary Creeek Plains? Hhmmm...
Seriously, you are going to be nased in Sheffield so I don't think you can go too wrong with any walk up past Lake Rowallen.
As for times and with a 4yo, I'm the wrong person to ask.
Have a read about the Lees Paddock track and Oxley falls.
Then there's Dove Lake circuit. Everyone has to do it once, and it's nice - and at this time of year, hopefully the crowds will be less.
There are some other shorter walks from the visitor centre at Cradle Mtn, both east and west, that are nice - enchanted walk on the west side - a bit shorter than what you had in mind but easy, flat, boardwalked, beautiful, but near a river so keep an eye on the littlie. Could be good as a top-up walk if you want to see more after Dove Lake circuit.
On the east side the walk is a little more challenging (for a 4yo perhaps) but really quite acceptable as an option - Dove Canyon track. It's a circuit but I've never done it - only been in starting at the bridge, got to some falls on a slight side-track, and returned.
Probably a better intro than Lees / Oxley - less exposed in the sense that you are closer to civilisation / car.
http://maps.google.com.au/maps?hl=en&ie ... 50511&z=15Best to check with the visitor centre at Cradle Mtn to get more detail on them. A good carpark there too, and facilities.
They may also have some other ideas.