Well, I have to say that I am not going to be purchasing seats with inbuilt harnesses for my children despite them falling into the category where they need one (5.5 and 3.5 years old, 16 and 17 kgs) despite both being too tall for the manufacturers recommendations of where the harness should sit. (no further than a few inches below shoulder level on all the ones I have seen)

it is WAY too expensive, I don't have that kind of cash.
Can I ask what are they sitting in now? And do you live in NSW? Cause even without the new laws, at that weight they should still be sitting in child seats.