Angus mentioned an online cycling magazine Momentum. The Sep/Oct 2010 edition looks at transporting children. On page 25 is a picture of an Xtracycle or long bike with 2 seats for small children attached. When the children grow, remove the baby seats and let them sit astride on the back. When the children are at school, use to carry your weekly shopping. http://momentumplanet.com
There is another children bicycle transport called Targa, with space for two children at the front. I have not checked the link of tagabikes.com/au
Another option is the trailer. I ride around with our 14 month old in a Chariot Trailer, pricey bit of kit but the cargo it is carrying is priceless. I started out using it when our daughter was 8-9 months old. Most of the time she loves it but is not so thrilled about the helmet. We did the Coast to Coast (Meadows start) in early March.
Yeah we have a Chariot trailer too, and our 2yo loves it. I started him on a child seat on the back of my recumbent trike at about 10 months (just sitting in it initially, then starting off very slowly), then we got the trailer. We ride him to child care in the trailer now and it's just a normal part of his routine.
The trailer has good protection against wind and rain too - it's all-weather. Can also carry toys, nappies, bags of stuff, water bottles etc. Highly recommended if you can afford it.
Some AC members commented that they are afforded more road courtesy when pulling a kiddie trailer. Not always the case, as one member had drink thrown on his trailer with children.
When the child grows out of the trailer, you can add-and-remove a hitch-bike, and in-between use the trailer to carry shopping. I read of an un-supported climber who rode from Europe to climb Mt Everest, with his climbing gear in a kiddie trailer.
We've used this type of seat for all three of our kids: http://www.bicyclestore.com.au/beto-deluxe-baby-seat.html and I have never had any complaints. All of them started at around 18 months of age and the were/still are using them up until the weight limit of 20kg (around 4 years). You can still talk to them, even if they are seated behind you, and they are protected from the wind (remember they arent pedalling so wont be as warm as you).
As for places to ride, if a kid likes sitting in a bike seat then just about anywhere is going to be interesting for them. You will pass dogs, trains, buses, parks, playgrounds etc etc. But any of the bike paths is a good place to start - along the Torrens from Henley Beach, the Mike Turtur bike path from Glenelg to the City, through the Parklands on the network of bike paths, along the beach on the Seacliff to Outer Harbour bike path. All good places to explore with a young child on the back/front/in a trailer.
Emily, an AC post today that Torpedo7 have on special child trailers. Order online from interstate, maximum delivery fee is $80, and delivery in a week.
Some AC members think the business is OK. On Monday I ordered from Torpedo7 for the first time, a channel rack to carry 3 bikes, also on special.