"samwich" is a bit weird for me to understand where did it come from hah! Does it exist ? It exists. It came from baby talk or an attempt to emulate baby talk. Some people think that it is cute. Seriously.

It all depends on how careful people are when speaking. "Sandwich" is a little tricky to pronounce carefully, although many people do. When spoken casually or carelessly, "sanwich," "samwich," "sammitch" are all possible. Perhaps the most common sloppy pronunciation of all is something close to "sa'wich," with the "a" retaining the usual sound, the "d" disappearing completely, and just the _slightest_ trace of something like an "n" or an "m." These are not really variant pronunciations, just careless pronunciations.

I heard that children use to say that

It's "sandwich", but when we are lazy we say "san-witch" Laurence