The verb "run" comes from the conjugation of the verb in infinite "run." "Run" is conjugated in the third person by referring to the children and in the present, since the children perform the activity at that time.
Unlike English, in Spanish when referring to the third person in the present tense if there is a change during the conjugation of the verb in infinitive. In this way the verb "correr" (infinitive) changes to "corren", that is, change the "r" to "n".
It depends on how you use it but it will most likely be "el" like if you're saying "I just got a new phone on Monday" it will be "Acabo de recibir un nuevo teléfone el Lunes", in spanish.