Learned the meaning of the final sentence

I posted on japanese.stackexchange.com asking for help understanding that sentence I couldn't translate and received a great response from a user named Chocolate who broke it down piece-by-piece. That sentence means "Why? ... I can't tell that to Mom.", so the first page says "Sachan ate breakfast urgently. Mom always says 'please ate slowly' but today it cannot be helped. Why? ... I can't tell that to Mom." Now that page one is done, I suppose it's time to move on to page two!

Page one of the Japanese children's book ふんふん なんだかいいにおい

The first page in the Japanese children's book ふんふん なんだかいいにおい that I'm trying to read 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。

The first sentence (Sachan wa ooisogi de asagohan wo tabemashita) means something like "Sachan ate breakfast urgently". I understood that without having to look anything up except I don't know why the particle で is there after おおいそぎ. I struggled more with the second sentence. I knew "ゆっくり たべなさい" meant something about eating slowly but I didn't recognize the ~なさい (~nasai) conjugation. Searching the web I found that it indicates a request, like ~kudasai, but is used when talking to someone who is a lower rank than you such as a parent to a child, so that phrase simply means "Please eat slowly". I also didn't know that "と いう" means "to say" in that a person was just quoted. I did know that けれど means "but/however", きょう means "today", and ゆっくり means slowly but I didn't recognize the なんて on the end of it. Apparently it's just a word that give emphasis. Then I also didn't recognize していられないの which according to Google Translate means "cannot be helped". So the second sentence should mean something like "Mom always says 'please ate slowly' but today it cannot be helped". The third sentence 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 "mom" but I didn't know ないしょ or なの. Apparently ないしょ means "secrecy" and なの is just an informal version of ですか, which is sort of like "to be?". But both Google Translate and Microsoft Translate give different translations of that sentence, neither has a word like secrecy in it and neither seem to make much sense in the context so I'm still not sure what that sentence means.

Happy New Year!

Happy New Year everyone! We're only two days in but I hope you all got off to a good start. We went to see Aquaman yesterday, which was fine. Enjoyable for the most part, Jason Momoma was great as usual. I loved his character in Stargate: Atlantis and enjoyed every performance I've seen since then, even when the movies themselves were nothing spectacular (Conan the Barbarian, Braven). He brings an energy that makes the character enjoyable to watch and easy to get invested. That's something the DC movie universe really needs. My eyelids started getting real heavy during that ending CG battle, though, when it switched from watching characters I care about to a whole bunch of computer generated people I had no investment in fighting each other. I struggled to stay interested during that whole sequence, but up until that part I was enjoying it. I thought they did a good job with the underwater effects, the hair and the way people moved.