We did the GVBR with 2yo twins last year, about to go around again this year, and then onto NZ. We are using the 2011 GVBR as training for the NZ ride.
We've got them in a trailer (a croozer), great for the littlies as they often go to sleep between brekky and the first rest stop and sometimes in the arvo, so dont get wet and can play with toys etc. The combined weight of the kids+trailer is at least 40kg, so hubby and I share the towing, change every 20km or so. We did have to put in a fair bit more training (compared to rides pre kids) to make sure we could handle the extra weight. Training with the trailer was mainly on bike paths, we dont have many bike lanes wide enough for the trailer near home (outer eastern melb). We ended up doing laps of Kew bvd for some on road training. I should probably mention that we are towing them with road bikes.
We stopped for longer than normal at rest stops and lunch to let them have a good play, and always had a good selection of snack food and small toys. I now know the location of the park/playground in every town we stopped at. The only slightly difficult time is trying to get the tent up while they are busy burning off some energy. Ours usually run about inside our tent while we put it up around them. If you are by yourself a sleep easy tent would make this a non issue.
Otherwise, on the ride itself we had a fantastic time. Everyone stops to chat to the kids, and they dont need too much entertainment with so much to see and do. They had a great time, despite the inclement weather (nothing some gumboots and a raincoat wont sort out). The bv staff went out of their way to make it easy for us (one of the kids has food allergies), we probably had a better experience than the 5+ rides prior to the kids.