Bread : You can use either wheat or white depending on your little ones preference. Never toast the bread, just send plain with or without crust.

Sauce : Just for little soft texture you can use any kind of sauce or spread. I usually send little mayo which goes well with sandwiches.

Stuffing : You can choose the veggies that you love, my picks are onions, mushrooms, sliced tomatoes, cooked chicken (left over will work, rotisserie chicken) and lettuce. In a pan heat oil and saute mushrooms and onions over medium flame with salt and pepper. When done you add the cooked chicken along with it and toss for few seconds. If using uncooked chicken then add the along with the veggies in the beginning, that way it will be cooked. Let this mixture cool a little bit.

Assembling : These ziplock compartment boxes are new in market and work great for lunch boxes something similar to bento.  I usually send them separate that way it doesn't become soggy by the time they open it. If you're kid is too young to assemble then its ok to stuff and send them as a complete sandwich.  Mayo is optional and you can also spread that in advance if you don't want them to mess. Sort the tomatoes and lettuce in one, cooked veggie in another and bread with mayo in the big section. Send this with pack of juice or a special dessert treat and I'm sure it would delight them. A little shredded cheese is ok too, if that cheers your child, which usually happens with my son.