Yes it seems you may be in a similar sistuation to us, I think becuase your visa has been issued and validated you will have to apply for a seperate visa for your youngest. As you are still in the UK you could apply for the 101 visa offshore which does work out cheaper $2k instead of $3k if you process onshore, but you would need to make sure the visa will be issued in time for your travel date of June 2013. This option would be better because if you process onshore as we are doing you become the sponsor for your child and waiver any government payments such as family tax/child care support etc for 2 years once you get the visa, and with the cost of living here you really need the payments especially if you are living on 1 wage as we are.
My daughter travelled into Australia on a tourist visa as this was recommended by our CO, I have since read that she could have been refused entry into the country so I dont think that I would advise to do it the way we have. Although we did a printed copy of the email to show border control if there was to be an issue.
I personally would seek advice from your CO, I would check 100% that therre is no way they can add your youngest to your visa, if not try for the offshore approach over onshore. Maybe seek the advise of an agent too.
If you need anything further let me know, feel free to PM me. Good luck!!