Hi,
The school year in which your children will enter depends on their birthdays, and different states have differing cut-offs. In UK 1st September is typical cut off, In Victoria it is end of April and I think in NSW it is end of July.
We took our daughter from UK school aged 7, and returned 6 months later. Moved again in Sept 2009 and back to UK Dec 2010.
In my opinion, the impact on schooling depends alot on the school. We felt in the first school that our daughter was a long way ahead, and this may have been due to her having been in reception since aged 4 (May birthday, so young for her school year) and moving to a year below the one she left in UK (becoming one of oldest in year), due to the cut off.
On our second period in Melbourne, we were much more aware and elected to go private for schooling. The school was fantastic and placed our daughter in the year above her age (so same year as she had been in, in UK).
Going private is not the only option to good schooling, but it gave us what we wanted. Placing children in years above their age group may be ok academically but I would be very careful especially if you are thinking in regard to teenage years etc. Many classes contain a variety of ages, as some parents hold back there children for a year. It could be for example, that a 12 year old (placed in a year above their norm), in a class of predominantly 13 / 14 year olds, could also have a 15 year old classmate. Academically they might be equals, but social maturity would be very different.
In our opinion, our daughter has lost nothing, and gained lots by travelling and changing schools. It has developed many skills, including the ability to cope with change and make new friends, but has also opened her eyes to a whole world of experiences.
Academically, she is still a well performing student. The only subject that she has lost a little ground on is LOTE (languages), where she studied Mandarin in Australia, and has had to learn German in her current UK school.
Hope this helps. Kids are far more resilient then you or me, the chance and adventure is a great opportunity, carpe diem!