I work in the optical industry so just thought I'd throw my two cents in
The eye test should be bulk billed to medicare. You are more then welcome to ask the optometrist for a copy of the spectacle prescription if you want to shop around. Depending on the prescription, kids glasses shouldn't be any more expensive then adults. The only exception is if the power is quite high and the lenses need to be thinned down to look nice. I work for specsavers and the kids frames are always included in the same deals as the adult ones, so 2 for 1 etc. And unless everyone in the family needs glasses, private health insurance extras often aren't worth it (unless you need it for a lot of other stuff, dentists, physic etc.).
Word of warning: don't buy spectacles off the internet. There is no quality control, especially if they come from overseas then they aren't covered by the same standards we have to adhere to (by law) in Australia. And a pair of glasses should always be properly fitted and adjusted by experienced optical dispensers, especially for little faces.
Good luck with it all