You could try to apply for a B2 visitor visa, and if you were denied only due to the ban, and not due to immigrant intent, and if the visa officer recommends a waiver, you can apply for a nonimmigrant waiver for the ban. Unfortunately, you are very likely to be denied for immigrant intent (for which there is no waiver), both due to your past overstay, and due to you having a US citizen girlfriend living in the US whom you want to marry.
If you and your girlfriend want to marry and have you immigrate to the US, there are two options. If she can leave the US and you guys can meet up (doesn't matter if it's Australia or a third country), you guys can get married and then she can petition you and your child to immigrate. You would need to get an waiver to be able to immigrate before the ban is over; and an immigrant waiver for this ban would require you to show that your US citizen spouse would suffer "extreme hardship" if you can't be in the US.
Alternately, she could petition you for the K1 fiance visa; that way, you can get the K1 visa, and then marry in the US, and then apply for Adjustment of Status in the US. Getting the K1 would also require an immigrant waiver, which again would require you to show that your fiance would suffer "extreme hardship" if you can't be in the US. However, K1 requires that you and your fiance have met each other in person at least once in the 2 years before she petitions you, and it sounds like you guys don't meet that requirement as you left the US 2 years ago.