Bitterballen aren't served to fans in any football stadium, except in the VIP areas maybe.
Snacks for the fans are the regular ones, normally, which are generally these: hamburgers, frikandellen, fries, kroketten and like Wally said the 'broodje worst'.
Like always there are exceptions too, like we've got the German 'currywurst' in Venlo
Once I personally did research about this subject for RKC Waalwijk, I still have got that, their exact assortment back in 2008 was like this:
Warm snacks: Frikandel, Kroket, Hamburger (all served with or without bread)
Cold snacks: Masterfoods
products like Mars bars, Snickers bars etc.
Drinks: Coffee, tea, chocolate milk and soft drinks like coca cola, fanta etc.
Main complaints by the fans were no fries and no beer
Another traditional Dutch stadium snack might be the 'broodje bal' btw, a meatball served with bread.
I like FourFourTwo btw, good magazine!