I have a white food kid. At the start of the year, that was all he ate, but with the other kids eating different stuff, he has begun to try a heap of different things.

Our kinder didn't actually allow home baked stuff, and certainly no biscuit/slices/cakes. Muffins were allowed but only little ones.

I sent in
squeezy yoghurt (I have the reusable pouches, but I find them totally gross to clean out)
cut up cucumber/carrot/celery
rice cakes
rice cracker/jatz and cheese/hummus
fruit cut up (can secure with a rubber band in it's original form to prevent it going brown)
sandwiches cut into small shapes (as a snack)

boring I know. i hate sending the same thing over and over. They however seem to prefer it.