Child visas lodged in the UK are quoted as taking approx. 12 months, although they can, and do, come through much quicker. The initial email to acknowledge receipt of your application can take a few months; remember someone has to enter all the information onto the Government system since these applications are not yet available through the ImmiAccount process.
You can help your application by ensuring all the required documentation is included with the application and even completing medicals and the relevant police clearance certificates.
If you have a genuine reason for the visa to be granted sooner (i.e. a job that you must start on a particular date) you can provide this information to AHC; they are not unreasonable people.
Everyone's situation is different and seeking professional assistance is always advisable.