Thanks everyone. You're all very kind. We already have her first overnight bushwalk booked in for late September, when we've hired out the Scout Hut at Cradle Mountain. It's only a 45 minute walk in, so we figure we can probably carry two kids and a bit of gear that far, and maybe take an hour or so to get there. If necessary, it's close enough for me to jog back to the car and pick up a second load of gear.
Hopefully there's still be some snow around too!
The only downside is that we'll have to carry out bags full of dirty nappies. I sure hope the older one is toilet trained by then.