I went to a do once where they had mini burgers and mini fish and chips, it was an adult thing but such a nice idea and sounds like it might go down well with the kids?
When I used to do children's parties when I worked at a fast food place I found squirty tomato ketchup was great to have around as they could draw stuff with it and make patterns and such like on their burger buns.
Could you get some ice cream cones and do that with different icecreams if the weather is nice? Get some stuff they can decorate them with like sprinkles and sauce.
Maybe make or buy a stack of cupcakes (or muffins) rather than one big
cake and put them on a tiered
cake stand if you have one or could borrow one, can look more arty than a
cake and, depending on how safe and easy to light and carry, could look amazing with candles in each.