It can be. -lar is optional in sentences like this, with a human subject that is already marked as being plural.
So both Çocuklar okuldan geliyor and Çocuklar okuldan geliyorlar are correct.
(On the other hand, if the subject were non-human, e.g. "The apples are falling from the tree", then you must not add the -lar ending to the verb: "Elmalar ağaçtan düşüyor".)
Infinitives involves verbs and not nouns. I am guess that you are talking about the accusative case. One noun can only take one case at a time :) (You can't say okul-u-nun-dan-da-ya-yla) :) Just one case per noun. Also, the accusative case is for specific direct objects of which there are none in this sentence.