We had to make ours the night before, I remember. That way you can't complain you don't like it! What if you sit down with them and make a list of everything they'll be happy to eat, and then they can sort of put their orders in at the start of the week or something? Apart from sandwiches, how about zucchini slice, scones, hearty muffins (like fruit and oat), puff pastry tarts (I chuck a bunch of stuff like feta, cherry tomatoes, bacon in the centre of puff pastry squares. Or apples or peaches or something). Even if you freeze batches of muffins (I just looked up a recipe for breakfast muffins, breakfast, lunch, same diff), they can just pull them out as needed.

Otherwise. Let them go hungry? I mean, are they not eating because they think it's yuk, or are they preferring to skip lunch to play and socialise? You could see if their teacher can make sure they eat? My kids have their lunch for afternoon tea if they don't finish it. But if they're just doing other stuff, the food is there if they decide they're starving, but perhaps even if they have exactly what they want they wouldn't eat it if they're busy? Don't even know if that helps, but i hope you figure something out so they have some brain food for the arvo. Would they have a smoothie if you could figure out a way to keep it cool?