Party bags, go to the pound shop they do packs of 6 bubble pots for £1, shove in a balloon, a whistle, a lollipop and anything else small like that, then pop in a bit of cake and that should be that. They have a party bit in tescos usually near the cards but maybe not suitable for a 1 year old.
As for the food, things like mini pizzas and cocktails sausages. Maybe a bowl of corn crisps like wotsits or something they're not going to have trouble with. Are they going to sit down and eat? If they are some teeny tiny sandwich triangles would be nice.