They're sharing one.
'Ci chłopcy mają kanapki' - would be either they have one (or more sandwich each)
"chłopiec" just means "a boy".
"guy" is a colloquial word. Maybe it could be considered to be a translation of "chłopak" = an 'older' boy, but definitely not for "chłopiec", who is a younger boy. And we usually don't accept colloquialisms anyway.