Copiii pleacă din parc acum.

copii = “children” (indefinite plural)
copiii = “the children” (definite plural).
Romanian “definite article” for masculine/feminine nouns in the plural is an enclitic -ii (or -le for some feminine nouns), attached to the noun.

pleacă is the 3rd person plural, present tense of a pleca (“to leave”).
Subject “they” (ei/ele) + plec- → pleacă.
Romanian doesn’t distinguish simple vs. continuous “they leave/they are leaving” in the verb form—both use the same present tense.

When you use din + a definite noun, you drop the noun’s article:
din + parc (definite article is built into “din parc”)
“dinul” would be din + al + parcul, but that’s ungrammatical here.
You simply say din + [indefinite noun] when expressing “out of/from the (specific) place.”

Yes, de la parc literally means “from the park,” focusing on origin or source.
din parc also means “out of/from the park,” but stresses movement out of its interior.
In practice they’re often interchangeable for leaving a place, though din implies exiting an enclosure.

acum (“now”) is quite flexible:
– Copiii pleacă din parc acum.
– Acum, copiii pleacă din parc.
– Copiii acum pleacă din parc.
All are grammatically correct; the position only shifts emphasis slightly.

You just use the simple present: pleacă.
If you want to emphasize “in the process of leaving,” you can add tocmai or și:
– Copiii tocmai pleacă din parc.
– Copiii pleacă chiar acum din parc.

Yes, depending on nuance:
– se duc (they go away) – more neutral or colloquial.
– părăsesc (they abandon/leave behind) – more formal.
– își iau rămas-bun (they take their leave) – literary.
But pleacă is the most common for “they leave/depart.”