You can use Wiktionary: https://en.wiktionary.org/w/index.php?search=kroj%C4%85
'krajać' is a habitual form that some verbs have. Its only translation can be Present Simple, so it doesn't work here. 'kroić' works well for both Present Simple and Present Continuous.
Frankly, most habitual verbs seem quite natural to me, but 'krajać'... I don't know if I would use it myself.
Ok, I was trying to find the infinitive of 'kroją' in Wiktionary and stumbled across 'krajać'. Dziękuję!
Because "nasze" is the neuter version of a possesive which means "our". So they aren't cutting just any random sandwiches, they are OUR sandwiches. How dare they.