Hi...
as for the sponsorletter requirements check this link...
http://www.bali-expat-business.com/indo ... sa.php?p=6
I have no idea how much 450 aussie dollars is, but for prices check this link
http://www.bali-expat-business.com/bali-product.php They mention that for groups there are different rates and I would think you would qualify as a group :) .
Don't know the ages of the kids, but if they are small kids it could be a hassle doing a visarun, but like Ronb said/wrote all persons would have to leave the country before the visa expires.
Hope everythings works out, take care, friendly greetings.....Gilbert.