I think this will depend on the region and the number of food allergies that the children have, so you will definitely want to check on that first. Considering that these are 2-3 year olds you're probably not going to get much in the way of adventurous eating either but here are some things our adventurous 3 year old ate:
Roasted Mushrooms, probably could do small stuffed ones
Maki Rolls, the ones we had were a variety of crab leg, avocado or cucumber but nothing spicy
Meringue Mushrooms (a sort of pain to make unless you are used to it but amusing)
Cucumber Sandwiches, no tea party is complete without!
I could probably think of a few more but this should get you started.
These are some of the items I've seen that children really go for:
Bananitos, if you can get them. These are teeny tiny bananas, the size of an adults' thumb. Very cute and kids love them.
Crazy Jelly/Jello. You make several different types of jelly and then scoop chunks out of, say, the strawberry, raspberry and blackcurrant and put them into an almost-set apple jelly, along with bits of strawberry, apple, blueberries or pears.
Marzipan Characters. Buy some marzipan for cooking and some natural food colouring. Roll the marzipan into balls of different sizes and make Lego-style people and animals. Needs some imagination and an allergy warning on the marzipan (made from almond paste)
Scotch Pancakes. These are small, but thick pancakes that usually have raisins too. They're best warmed, but kids love them.