We have been to a party with only choc spread sandwiches. I did notice (most parties round here are a bit OTT on the healthy stuff), but I would not judge. Parties are hard work, esp for working parents trying to cater for a zillion different dietary requirements!
My personal experience is that kdis don't eat the sandwiches that well, no matter whT the fillings are. Why not do other savoury snacks that are no effort? Various frozen vegan (which tend to fit kosher/halal requirements) 'sausage' rolls available which all got eaten when i did them, also pre cooked pizza (if no kitchen available, can leave cheese off for vegan or use vegan cheese, avoid pork salami etc).
If yuo want to do minimal effort sarnies I would go for cheese, houmous and jam if you want. Use dairy free spread. Check bread for milk products and other requirements, That should cover everyone.
If anyone comes up with a really difficult diet ( I had a dairy and gluten free vegetarian kosher boy one year) - text parents and ask what they like. I then did this boy his own sandwiches on plate, and he could choose which other bits to have. Parents bought him a little cake from home instead of bday cake.
In party bags, i've found yo yo bears seem to fit all diets and almost satisfy haribo-allergic yummy mummies.