We find smoothies filling enough, especially with the addition of good fats like coconut oil and avocados. Granted, we might need a snack a couple of hours later, but that's ok with me. We had smoothies with breakfast this morning, and I tried grating frozen grass-finished beef liver in them....no one noticed! I also sprinkled some flax seed meal in them for more fat. (We eat a lot of fat around here, if you can't tell!)
If you find your children still hungry after one sandwich, my question would be how are you preparing your bread? Unfortunately, loaves of bread from the store are empty carbs and improperly prepared grains. Grains can be made much more filling simply by preparing ahead of time through sprouting or sour leavening. I find it difficult myself to eat an entire sandwich made with my homemade sourdough. And my slices are about 1/2 the size of store bought slices!
I agree w/ Tristan as well...give those kids some fiber in the form of raw and cooked veggies at lunch. When we have spaghetti or other pasta for lunch, I will frequently throw together a salad to go along or cook some broccoli and douse it with good olive oil.
The snack lunches are filling as long as they're not just raw fruits and veggies with nothing else. The great thing about snack lunches is that you can tailor them to each child's preferences without adding extra work for yourself. For example, one of my children really likes celery with PB; the other one likes to dip carrot sticks in PB. Easy to give them each what they like without creating extra work. Sometimes one wants yogurt and the other just wants a glass of raw milk to drink. Easy. These snack plates are filling when high fat foods are added such as PB or almond butter, sliced avocados or guacamole, raw cheese, bread and butter, sour cream, etc.
Typing about all this good food is making me hungry!