What does "なぜって‥‥‥ それは おかあさんには ないしょなの" mean?


I've been learning Japanese through phone apps and am now trying to read a
Japanese children's book called ふんふん なんだかいいにおい. The first page is:

> さっちゃんは おおいそぎで あさごはんを たべました。 おかあさんは いつも 「ゆっくり たべなさい」 と いうけれど、 きょうは ゆっくりなんて
> していられないの。 なぜって‥‥‥ それは おかあさんには ないしょなの。

I think the first two sentences mean:

> Sachan ate breakfast urgently. Mom always says "please ate slowly"' but
> today it cannot be helped"

I can't figure out that the last sentence. It starts with なぜって‥‥‥ which I
didn't recognize at all and apparently just means "why". Then それ means "that"
and おかあさん means "mother" but I dodn't know ないしょ or なの. Apparently ないしょ means
"secrecy" or "privacy" and なの is an informal version of ですか?". Both Google
Translate and Microsoft Translate give different translations of that
sentence, neither has a word like secrecy or privacy in it and neither seem to
make much sense in the context so I'm still not sure what that sentence means.




> なぜって‥‥‥ それは おかあさんには ないしょなの。

_lit._ "(You ask me) why? ... That (=the reason) is secret from Mom."  
"Why? ... I can't tell that to Mom. / I can't tell her why."

Breakdown:

  * それ ("that") refers to なぜ ("why/reason"). 
  * なぜ is referring to the reason for きょうはゆっくりなんてしていられないの ("I can't take my time today. / I can't be eating slowly today.") 
  * ないしょ is a noun meaning "secret". 
  * The な in なの at the end is the attributive form of the copula だ. だ conjugates to な when followed by の. 
  * の is a sentence-ending particle used in casual (and often childlike or feminine) speech. 
  * The って is an adverbial particle used to repeat other's word and talk about it. From 明鏡国語辞典: 

> って ㊁ 〘副助〙➋相手の言葉を繰り返し、それについて述べる意を表す。「誰が犯人か **って** 、あいつに決まってるよ」「やりたくない **って**
> 、どういう意味？」




> なぜって‥‥‥ それは おかあさんには ないしょなの。

_lit._ "(You ask me) why? ... That (=the reason) is secret from Mom."  
"Why? ... I can't tell that to Mom. / I can't tell her why."

Breakdown:

  * それ ("that") refers to なぜ ("why/reason"). 
  * なぜ is referring to the reason for きょうはゆっくりなんてしていられないの ("I can't take my time today. / I can't be eating slowly today.") 
  * ないしょ is a noun meaning "secret". 
  * The な in なの at the end is the attributive form of the copula だ. だ conjugates to な when followed by の. 
  * の is a sentence-ending particle used in casual (and often childlike or feminine) speech. 
  * The って is an adverbial particle used to repeat other's word and talk about it. From 明鏡国語辞典: 

> って ㊁ 〘副助〙➋相手の言葉を繰り返し、それについて述べる意を表す。「誰が犯人か **って** 、あいつに決まってるよ」「やりたくない **って**
> 、どういう意味？」

