Contamos la aventura al niño.

It can be either, depending on context.

• Present: we tell
• Preterite: we told

For -ar verbs, the nosotros form is the same in the present and the preterite: contamos.

So this sentence could mean either:

• We tell the adventure/story to the boy
• We told the adventure/story to the boy

Usually, other words in the sentence or the wider context make the time clear.

Here it means to tell or to narrate.

Contar has more than one common meaning:

• to count: Contamos hasta diez = We count to ten
• to tell / narrate: Contamos la aventura = We tell the adventure/story

In this sentence, because the object is la aventura, the meaning is clearly to tell/narrate, not to count.

• la aventura = direct object
• al niño = indirect object

Why?

The direct object is the thing being told: the adventure/story.
The indirect object is the person who receives it: the boy.

So the structure is:

• Contamos = verb
• la aventura = what we told
• al niño = to whom we told it

Because niño is the recipient of the action, so Spanish uses a for the indirect object.

In English, we often use to:

• We told the story to the boy

In Spanish, that to is usually expressed with a:

• Contamos la aventura al niño

So here a does not mean movement toward a place. It marks the person receiving what is told.

Because a + el contracts to al in Spanish.

So:

• a + el niño → al niño

This is a normal rule:

• Voy al parque = I go to the park
• Damos el libro al profesor = We give the book to the teacher

The main exception is when el is part of a proper name, such as El Escorial:

• Voy a El Escorial (no contraction)

Not exactly in function, even though it is the same preposition a.

Here, al niño is an indirect object: the person who receives the story.

That is different from the personal a, which is used before a direct object that is a specific person:

• Veo al niño = I see the boy

In Contamos la aventura al niño:

• la aventura is the direct object
• al niño is the indirect object

So learners may recognize the same word a, but it is doing a different job here.

Because the sentence refers to specific things:

• la aventura = the adventure/story
• el niño = the boy

Spanish often uses definite articles where English does too, and sometimes even more often than English.

If you changed them, the meaning would change:

• Contamos una aventura al niño = We told a story/adventure to the boy
• Contamos la aventura a un niño = We told the story to a boy

So the articles help show whether the nouns are specific or not.

Very often, yes — and many speakers would find that version more natural.

Spanish commonly uses an indirect object pronoun even when the full noun is also present. So you will often hear:

• Le contamos la aventura al niño

Here:

• le = to him
• al niño repeats or clarifies who that him is

Without le, Contamos la aventura al niño is understandable, but Le contamos la aventura al niño is often more idiomatic in everyday Spanish.

Yes. Spanish word order is more flexible than English word order.

These are all possible, depending on emphasis:

• Contamos la aventura al niño
• Le contamos la aventura al niño
• Al niño le contamos la aventura

The basic meaning stays the same, but the focus can shift:

• Al niño... puts early emphasis on the boy
• la aventura after the verb is the neutral pattern

Spanish uses word order partly for emphasis and information structure, not just strict grammar.

You can replace each object like this:

Replace the direct object

• la aventura → la
• La contamos = We tell/told it

Replace the indirect object

• al niño → le
• Le contamos la aventura = We tell/told the story to him

Replace both

• Se la contamos

Why se and not le la?
Because Spanish changes le/les to se before lo/la/los/las.

So:

• Le contamos la aventura al niño
• Se la contamos

This is a very common pattern in Spanish.

Because contar is very natural when you mean to tell a story, to narrate, or to recount something.

Compare:

• contar una historia / una aventura / un cuento
  = to tell a story / an adventure / a tale

• decir algo
  = to say something

So with la aventura, contar fits very well because the idea is narrating it, not just saying a few words.

For example:

• Le contamos la aventura al niño = We told the story/adventure to the boy
• Le dijimos la verdad al niño = We told the boy the truth

Both verbs can translate as to tell, but they are used in different kinds of contexts.