This is called an accusative with infinitive construction, or accusativum cum infinitivo in Latin. We actually have them in English as well, though it's unclear how much of that is borrowed from Latin: "search your feelings, you know it to be true" (or less archaically, "she expected him to do that").
The way I usually think about them, the verb is taking two objects: a noun (the person being seen), and a verb phrase (the action being seen). The noun is put in the accusative, and the verb phrase is put in the infinitive, because that's the form things take when they're used as direct objects.
In other words, Marcus sees (Mārcus videt) Quintus (Quīntum) fall to the ground (ad terram cadere). The subordinate clause's verb goes into the infinitive so that it can act like a noun, which is the primary usage of the infinitive. And Quintus goes into the accusative because it's primarily the object of videt, even if it's also acting as the subject of cadere.
Notably, this can lead to ambiguity, since any object of the subordinate verb also takes the accusative case. This was used in a legendary prophecy from an oracle to Pyrrhus: "ajō tē, Aeacidā, Romānōs vincere posse". Since both the object of ajō "I declare" and the object of vincere "to conquer" are in the accusative, this can mean either "I declare that you, Aeacides, can conquer the Romans" or "I declare that the Romans can conquer you, Aeacides". If this ambiguity gets unacceptable, there are other ways to make subordinate clauses that avoid them. But they often require other verb forms that haven't been introduced yet at this point of LLPSI.
For more information, see here.