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.