I gave up on kids bags and went to the surf shop.  All our kids have billabong bags.  Our kindy boy has a small billabong bag that I got for our eldest 8 years ago.  They cost a bit more but have lasted forever.  DD got a billabong one when she was 5 and is 10 now and off to intermediate next year and shes only getting a new one cause hers is prob a bit small.  Well worth the expense in the beginning for us.