This isn't an original idea...or even a very new idea. Just one I had kind of forgotten about until I was thumbing through an early childhood activities book I'd had in college. So, you've probably seen it. But just in case you haven't or you forgot also and you've got kiddos home from school for lunchtime now, I'll give you a reminder too. : )
Fill small bowls with a tiny bit of milk and add a drop or two of food coloring. Mix it up. Give your kids bread slices and a paintbrush. Let them have at it. The colors show up nice and bright. Obviously they can't get them too terribly soggy or the bread won't ever recover. We've never really had too much trouble with that though.
When their creation is complete move on to sandwich mode. London wanted a pb&j so I toasted hers a little under the broiler so that it wasn't damp anymore and made her sandwich. Taryn wanted grilled cheese so I just spread the butter over her painting and grilled it up. The color still shows through.
It always seems they eat better when we do this. No bribing needed. Simple and fun!