I often don't make DD a sandwich as such but instead give her bits and bobs in various compartments in her dish - so bread and butter in one, chicken in another, tuna, tomato, sweetcorn, cheese, avocado, apple, grapes etc. etc. So not all those at once but some combo of usually bread, meat or tuna and some veggie or fruit.

That way you don't have to worry about the sandwich collapsing when they pick it up.