We did -

4 nights Sydney
flew up to Hervey Bay for a night, then out to Lady Elliot Island on the Great Barrier Reef, 3 nights (wonderful eco-resort, I swam with a shark - not on a tour, just there in the sea...)
Drove down to airport on the Gold Coast, stopping in Eumundi on the way for a couple of nights (lovely converted train carriage B&B) flew to Melbourne,
3 nights Melbourne,
Great Ocean Road for about 10 days -stopping several times, (apollo bay, warnambool)
Up to Hall's Gap, then back to Melbourne via Ballarat,
From melbourne back to sydney for 3 nights. Plenty to fill a month - Oz is a BIG place, so we decided to focus on the SE and not explore the centre etc (we also went in Jan when the centre would have been HOT, and we decided not to go with very young kids).

We drove, and did internal flights a lot. DH likes driving so that wasn't an issue, and we broke the journey a lot, another reason we probably did less than you could do in the time.

with those ages I would pootle around europe, otherwisec you'll be stuck in the car a lot. V envious!

you have to do Sydney, the Red Centre and Barrier Reef. They are the MUSTS. I've not done Perth but friends say it is good but a long way to drive.
Definitely don't drive there, maybe fly from Sydney.
The drive up to the Red Centre is nice but not for everyone so you might prefer internal flights which were pretty cheap when I went there.

Another vote for California here.

Of course they're hospitable to kids and actually most people eat early there so you'd be fine.

Great climate

Yosemite, beaches e.g. Santa Barbara, Santa Monica, Grand Canyon, LA for theme parks, San Diego etc etc

I'd live in California if I could.

We are really leaning towards Oz now. What itinerary would people suggest for a five week trip?

RebeccaJ - did you hire a camper or fly between places? We don't want hours of driving each day.

I'd be tempted to do Oz as beautiful at that time of year. I spent 6 weeks there many years ago (pre little darlings) and travelled from Melbourne, up to the Red Centre, Alice Springs, Ayers Rock etc, up to Darwin then across the Tablelands to Cairns and then spent a few weeks travelling down the gold coast and island hopping til I got down to Sydney. It was fab.
I'd be tempted to do it again now with our kids (16, 13, 3 and 7 months)

If you feel that is too far then America would be good. We hired an RV last year and drove from Orlando down to Miami then down the Florida Keys and up to Naples, Tampa etc. Our only mistake was not having a big enough van for all of us.

NZ and probably a week in Oz

norway

We had exactly this dilemma 2.5 years ago, and with a 8month old and a 2.5 yr old!

We did - Sydney, Great Barrier Reef, Great Ocean Road, Gold country, Melbourne, and back to Sydney. Fabulous, and plenty for a month!

I'd piss off on my own and leave dp with the kids ;-)

Seriously got to be OZ and NZ for me, probs in a camper van, with astop over in japan on the way