Hi Debs,
If you go onto
Department of Education, Training and the Arts it will tell you at what date they have to turn a certain age to get into that year. If you are coming out in Aug I think your son will continue in Year 7 until the January when he'll go into Year 8. I think yoru daughter will start Prep Jan 09 - but not 100% sure.
This website has links to all the schools websites in Queensland and is very good. My OH came out here in August and had a look around 3 primary schools here - he e-mailed the schools directly beforehand and all of them were really happy to show him around and explain the enrolment process. The schools own websites were really useful.
Hope this helps.
Liz