Shouldnt be a problem Clare - just enrol them when you arrive and let them go through with their age peers.
When you said you would be arriving end of summer - your summer or ours?
The best thing for your daughter will be to be sure she enrols to get the last two complete years of school in here because those two years form a composite course which builds to an end of year 12 assessment and determination of a university admission score (called various things in different states). To get into dentistry she is going to need a score 98+ which is tough and many kids do a first degree then go into medicine and dentistry postgraduate. So, if you arrive about August time, try and enrol her in year 10 for the rest of this year so she can begin year 11 at the end of January. If you arrive end of our summer - say March 2009 then you might be lucky and get her into year 11 without missing too much.
If you are here on a
PR visa then you wont pay fees in govt schools, in Qld, temporary business visa holders dont pay either but if you are here on a student visa then your dependents will pay international student fees ($10k pa+++).
At university, if you are
PR then you have just the local fees to pay but she will not be entitled to any fee relief or deferrment - fees are payable up front and dentistry fees are quite expensive.
Your son wont be a problem at all - he wont have to start school again, he will just slot in with whatever year his age peers are at. I doubt he will suffer at all, the general consensus is that kids from UK are ahead of Aussie kids the same age.