Alot of the time with older DC it is more about the socialising bit than the actual eating.
Like a PP mentioned getting them to make they're own popcorn it's something fun and unusual for them to do(not many children get to make things like that at home)and it's very very cheap and they can flavour it with different toppings,sugar,salt,maple syrup,a light sprinkling of a spice.
Cheap packets of biscuits that they can top with icing(rich tea,digestives)and sprinkles,it would be something fun for the younger DC to take part in as well.
Supermarkets own brand flour and margarine and jam is a very cheap way of making jam tarts/jam sponge.
They could make they're own pizzas using cheap wraps or the crusts of a loaf of bread(crust bread pizzas)they just toast the crusts lightly,cover with some tomato puree and a sprinkle of cheese and some ham or chicken or vegetables and then stick them under the grill for a few minutes.
Really cheap large ice pops(stick loads in the freezer)were always popular with mine and they're friends.
I know what it's like I was a single mum to 4DC and they always had lots of friends around and with doing the things above I managed to be able to let them all have snacks for a few pounds a week.