Lunch box sandwiches dont have to default to ham and cheese or peanut butter and jelly.
There are plenty of options to mix things up and keep them interesting. Ditching the sliced bread is a good place to start. Consider making sandwiches on a piece of baguette (or multiple smaller sandwiches on thin baguette slices). Or if crusty bread isnt your kids thing, try folding or rolling the fillings in a piece of lavash or naan flatbread.
Also get creative with fillings. Sliced meats can be combined with yogurt or cream cheese spreads, and cucumber sandwiches can be made more nutritious and filling by adding hummus.