Miaj gepatroj ne restos longe, ĉar ili devas reveni al la urbo vespere.

Because both words are plural.

• gepatroj = parents (plural, ending in -j)
• The possessive adjective has to agree in number with the noun:
  • mia patro = my father
  • miaj gepatroj = my parents

So miaj is plural because it describes a plural noun.

ge- shows that a group contains both sexes together.

• patro = father
• patrino = mother
• gepatroj = parents, meaning father and mother together

You will often see this with family words:

• fratoj = brothers
• gefratoj = brothers and sisters / siblings
• filoj = sons
• gefiloj = sons and daughters / children

restos is the future tense of resti.

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / jussive
• -i = infinitive

So:

• resti = to stay, remain
• restas = stays / are staying
• restis = stayed
• restos = will stay

In this sentence, ne restos longe means will not stay long.

In Esperanto, ne normally goes before the word or part of the sentence it negates. Here it negates the verb:

• ili restos = they will stay
• ili ne restos = they will not stay

That is the standard placement.

Because longe is an adverb, and it describes how long they will stay.

• longa = long (adjective, describing a noun)
• longe = long / for a long time (adverb, describing a verb)

Here it modifies restos, so the adverb form is needed:

• restos longe = will stay for a long time / will stay long

Esperanto often uses an adverb where English may use an adjective-like expression.

ĉar means because.

It introduces the reason:

• Miaj gepatroj ne restos longe = My parents will not stay long
• ĉar ili devas reveni al la urbo vespere = because they have to return to the city in the evening

Unlike some conjunctions, ĉar does not change the word order. The clause after it usually keeps normal Esperanto order.

Yes, gepatroj has already been mentioned, but Esperanto often uses a pronoun just as English does:

• Miaj gepatroj ... ĉar ili ...
• My parents ... because they ...

ili means they, and it refers back to miaj gepatroj. This avoids repeating the noun.

After a modal-like verb such as devi (must / have to), the next verb stays in the infinitive form -i.

So:

• devas reveni = have to return
• not devas revenas

This works like:

• volas iri = wants to go
• povas veni = can come
• devas labori = must work

Only the first verb is conjugated; the second remains infinitive.

iri means to go, while reveni means to come back / return.

• iri al la urbo = to go to the city
• reveni al la urbo = to return to the city

So reveni shows that they are going back to a place, not just going there for the first time.

al means to, showing direction toward a place.

• urbo = city
• la urbo = the city
• al la urbo = to the city

This is used because reveni involves movement toward a destination.

A useful contrast:

• en la urbo = in the city
• al la urbo = to the city

So here the sentence is about going back to the city, not being in the city.

In Esperanto, la is used much like the in English when the place is definite or understood from context.

• al la urbo = to the city

Without la, al urbo would sound unusual in normal Esperanto. In this sentence, the speaker likely has a specific city in mind, so la urbo is natural.

vespere means in the evening or during the evening.

It is an adverb from vespero (evening):

• vespero = evening
• vespere = in the evening

Esperanto often uses adverbs of time this way:

• matene = in the morning
• tage = by day / during the day
• nokte = at night
• vespere = in the evening

So reveni al la urbo vespere means return to the city in the evening.

Yes. Esperanto word order is fairly flexible, as long as the meaning stays clear.

For example, these are all possible:

• ili devas reveni al la urbo vespere
• ili devas vespere reveni al la urbo
• vespere ili devas reveni al la urbo

The original order is very natural and straightforward.

Not especially; it follows very normal Esperanto order:

• Miaj gepatroj = subject
• ne restos = verb with negation
• longe = adverb
• ĉar = conjunction
• ili devas reveni = second clause with verb phrase
• al la urbo = direction
• vespere = time

So the sentence is a good example of standard, natural Esperanto structure.