I am Australian and my 4 year old son is a US citizen. We are travelling to Australia VERY soon and I just realised he needs an Australian Passport. I have his Australian Citizenship certificate can I use that to leave the USA and enter Australia?
I have dual citizenship, and when I was going through immigration after arriving in Australia, I handed over my shiny new Canadian passport and was told that they couldn't let me in "with this" - so I handed over my Aussie one and it was all fine. I was travelling on a one-way ticket and I'm pretty sure that was why. If you're travelling on a return ticket then I think it's much less of an issue, but if you are settling then you may need to get him a passport because I don't think that they allow certificates of citizenship as adequate proof. They take about three weeks to issue a passport, not too bad.
Another thing to note when travelling with children (and without one of their parents, if this applies) is that you will need an express letter of consent from the absent parent stating that you are allowed to travel with the child.