I assume you'll be cooking for the kids but parents will be able to help themselves too?
Does the hall have a cooker? For dd1s 3rd we had a combination of sandwiches and pizza cut into small pieces. It depends how much time you have and when you have it. I find sandwiches time consuming beforehand and I don't usually have the time beforehand to make many sandwiches and dh seems unable to butter bread! but during the party I do have time to put in a couple of plain pizzas.
My two dds are good eaters at parties so I always do too much food and have lots left over as I assume everyone's like them. I find 2 sandwiches is more than enough for each child. I have also found kids love cocktail sausages. I always like to give them fruit and veg. Cucumber is quick to prepare, cherry tomatoes can be cut in half quickly as can grapes, strawberries. Blueberries just need to be washed too. Carrots seem popular with the children I know but I find peeling them can take a long time and the ready cut ones never taste very fresh.
For other nibbles kids seem to love chocolate fingers and iced gems - just as I used to as a child! Also Pom bears are popular although there's a mumsnet joke about Pom bears from before my time so I don't know if my suggestion has connotations!
I would say you probably don't really need sandwiches...like you say most people will have had lunch and be going home for tea, unless you are serving the food quite late so it will basically be tea.
My Ds1 is going to be 6 soon and his party will be 2.15-4.15 and I am planning to just do pizza, mini sausages, crisps, veg stick and dips and then those choc easter nests with the chicks and mini eggs in as it will be Easter weekend. I think that will be plenty as it is really just a snack and in my experience most kids aren't really that interested in food at parties!
Is it mostly adults you are having? If it is I would maybe just do some pizza, mini sausages/sausage rolls and maybe actually a cheese board then some cake.