My DD is does not really eat sandwiches, so I am wondering can you give Callum something different for lunch. Maggie often prefers to have hummus with carrot, cucumber, capsicum for dipping or a hot meal, such as a veg and rice dish. Even at 18 months Maggie will often chew something for awhile and spit it back out.

As long as they are happy and healthy I wouldn't worry too much.