لا الولد ولا البنت فهموا القصة من اول مرة، فانا قريتها لهم تاني.

Because this is the paired structure لا ... ولا ..., which means neither ... nor ....

That is different from the other لا that learners often meet early on, the one used in patterns like لا رجلَ meaning there is no man. That other لا has stricter grammar rules. Here, لا is simply part of a coordination pattern, so using definite nouns like الولد and البنت is completely fine.

It means neither ... nor ....

So:

• لا الولد ولا البنت = neither the boy nor the girl

Literally, it feels like not the boy and not the girl, but the natural English equivalent is neither the boy nor the girl.

Because the negation is already carried by لا ... ولا ....

The sentence is already saying:

• Neither the boy nor the girl understood...

So Arabic does not need another negative marker on the verb. In other words, لا الولد ولا البنت فهموا already gives the full negative meaning.

Because the subject refers to two people together: the boy and the girl.

A few important points:

• فهموا is plural because the sentence is talking about both of them together.
• It is masculine plural because Arabic normally uses masculine plural as the default when the group is mixed male + female.
• Egyptian Arabic does not normally force a special dual form in a sentence like this, especially when you have two separate nouns joined by ولا.

So even though there are only two people, فهموا is the natural Egyptian choice.

It means the first time, on the first try, or from the first reading/listening, depending on context.

Literally it is something like:

• من = from
• اول مرة = first time

But as a whole expression, من اول مرة is idiomatic. In this sentence, it means they did not understand the story the first time around.

The فـ means something like so, then, or therefore.

Here it connects the two parts of the sentence:

• They didn’t understand it the first time,
• so I read it to them again.

So فانا is basically so I...

Yes, أنا is optional here.

The verb قريت already tells you the subject is I, so Arabic does not need to say أنا unless the speaker wants a little extra clarity, emphasis, or smoother flow.

So both are possible:

• فأنا قريتها لهم تاني
• فقريتها لهم تاني

The version with أنا can feel a bit more explicit or contrastive: so I read it to them again.

قريتها breaks down like this:

• قريت = I read
• -ها = it (feminine singular)

So قريتها means I read it.

The -ها refers back to القصة, because قصة is a feminine noun in Arabic. Arabic very often attaches object pronouns directly to the verb, instead of using a separate word for it.

Because the sentence has both:

• a direct object: it = the story
• an indirect object: to them = the boy and the girl

So:

• قريتها = I read it
• لهم = to them

Together:

• قريتها لهم = I read it to them

This is very similar to English read it to them.

Here تاني means again.

So:

• قريتها لهم تاني = I read it to them again

Yes, you can also say مرة تانية, and that is very common too:

• قريتها لهم مرة تانية

Both are natural in Egyptian Arabic.
Very roughly:

• تاني = again / another time
• مرة تانية = one more time / again

This is very common in informal Egyptian writing.

A few things are happening:

• انا is often written instead of formal أنا
• اول is often written instead of formal أول
• Colloquial verbs are often spelled closer to how Egyptians actually say them, so قريتها reflects spoken Egyptian

In more formal Standard Arabic spelling, you would expect forms like:

• أنا
• أول
• قرأتها for I read it

But in Egyptian colloquial writing, people often simplify hamza spelling and write the spoken form more directly. So the sentence looks informal and natural for Egyptian Arabic, not fully formal MSA.