For picky preschoolers you may need to experiment so they will be inspired to eat their snacks at school. If you put some designs and styles, I am sure they will be delighted to take the snacks that you bring for them.
For beginners you may like this snacks for your preschoolers. You just need cucumber, carrots, and peppers cereals like frootloops. To make flower and butterfly sandwich, peel your cucumber for the butterfly antenna, top with a carrot slice and put the ring shaped cereals on top of the sandwich. Shape the sandwich that would look like a butterfly.
Shape another sandwich for a flower and decorate it with red peppers. You can make the stems, leaves and grass for the cucumbers.