En la feria hay un coro de niñas y niños que canta canciones tradicionales.

Hay is used to say there is / there are to introduce the existence of something: hay un coro = there is a choir.
Estar (e.g. está, están) is used for the specific location of something that is already known or identified:

• El coro está en la feria = The choir is at the fair. (we already know about “the choir”)
  Here the sentence is first introducing the choir, so Spanish uses hay.

The subject of the verb canta is un coro, which is singular, not niñas y niños.
Grammatically, the structure is:

• un coro (subject, singular)
• de niñas y niños (a prepositional phrase describing the choir; not the subject)
  Because coro is singular, the verb must be singular: un coro … que canta.
  If you changed the subject to las niñas y los niños, then you’d say que cantan.

Here que is a relative pronoun meaning that / which / who.
It refers back to un coro (the choir), so the phrase means:

• un coro … que canta canciones tradicionales = a choir … that sings traditional songs.
  Even though niñas y niños are people, the que here still grammatically refers to coro as the subject of canta.

Spanish uses the simple present much more often than English does to talk about actions happening now or habitual actions.
So canta canciones tradicionales can mean either:

• sings traditional songs (in general), or
• is singing traditional songs (right now), depending on context.
  You can say está cantando to stress “right now”, but canta alone is already natural and common.

In Spanish, when you talk about what someone does in general (an activity, profession, habit), you often omit the article.

• Canta canciones tradicionales = sings traditional songs (as a type of music).
  Adding an article changes the nuance:
• canta las canciones tradicionales = sings the traditional songs (specific songs already known to speaker and listener).
• canta unas canciones tradicionales = sings some traditional songs (an unspecified selection).
  Here the idea is general, so no article is the most natural.

Yes, Hay un coro de niñas y niños en la feria is perfectly correct and common.
Both orders are fine:

• En la feria hay un coro… (emphasis on the location: At the fair, there is…)
• Hay un coro… en la feria (more neutral; first mention the existence of a choir, then where).
  Spanish word order is flexible, and both versions sound natural.

De los niños would usually mean “of the children” in a more specific sense, like the children from this school or those particular children.
De niñas y niños describes the type of members the choir has (its composition), not “children belonging to” someone.
So:

• un coro de niñas y niños = a choir composed of girls and boys.
• un coro de los niños = a choir of the children (already identified children, more specific and a bit different in nuance).

In Latin America, feria can mean:

1. A fair or festival with events, games, performances, etc.
2. A street market / open-air market (in some countries).
   In this sentence, because there is a choir singing traditional songs, it most naturally suggests a fair or festival.
   Using en la feria for a local fair or community festival is very common in Latin American Spanish.

Yes. Un coro infantil means a children’s choir and is very natural and common.

• un coro de niñas y niños highlights explicitly that it includes both girls and boys (inclusive wording).
• un coro infantil focuses on the age (children’s choir) without listing genders.
  Both are correct; it’s just a difference in style and emphasis.

If you want several choirs, you change the noun and the verb to plural:

• En la feria hay coros de niñas y niños que cantan canciones tradicionales.
  Here:
• coros is plural,
• que cantan is also plural to agree with coros.
  The rest of the structure remains the same.