Merri Creek, then you can make it from the Capital City Trail all the way to Docklands (athough, there is a hairpin and a very steep downhill section just near Flemington Bridge station).
There's then a bike path that heads from South Melbourne to Port Melbourne. The tricky bit is getting from Docklands to South Melbourne. There's not really an easy way, without some road and/or footpath riding.
Thanks Zed, thinking that might work best perhaps up Canning St, a bit of footpath work (have kids on board!) and then down to Capital City trail near Birrarung Marr then on over to Sandridge Trail which goes along the light rail by the looks of it. Thanks for response!
An awesome day Cory, not too hot. Recovering from the big ride today, it ended up taking me almost 2 hours each way! I'm verrry slow but we had a great ride through the city which slowed things up but nice to ride through with the kids on board. Thanks for the tips and have a great week!