I think crisps would be fine, and a wonderful gift. Last time I visited oz I took over a variety of biscuits, crisps and teabags to perk up my homesick hosts . I think the only things they won''t let in are fruit, veg and plants and (because of disease or parasites); meat, dairy and honey (in case of transmitting viruses such as foot & mouth).
If, when you contact customs, they tell you it's a definite no-no then it would still be worth looking for an expat website or shop near you son who you may be able to order crisps from. When I lived overseas I used to visit such a shop and pay £1 a piece for a curly wurly (and this was in 1996!)