Kids say the darndest things but for how long?

Tonight after I finished reading to my newly 5 year old daughter, she watched as I put her books back on the shelf.  Every night the girls get 3 stories (that usually makes the 20 minutes you are supposed to read to your child).  Lately every time I read to her she makes sure to choose Green Eggs and Ham by Dr Suess.  As I was fitting them on the shelf a new one she got for her birthday didn't fit and I just put it faced out on the shelf.  Katherine (the 5 year old) then said "Mommy, are you trying to hide Green Eggs and Ham?"  It wasn't so much what she said but how she said it - as though she caught me trying to be sneaky and was warning me it wouldn't work. I had to laugh and my husband also laughed when I told him what had happened.
As is often the case, I started to think back at other funny things my daughters have said.  I still smile when I think of those moments.  Sadly, I am seeing more and more that they are growing up.  I know it needs to happen and am happy that they are developing as they should.  Part of me just mourns because I'm not sure how much longer I have to hear these humorous thoughts of theirs.  At what point will they no longer have the accidental jokes or cute misspoken words?
Just 2 years ago as Katherine was turning 3 she referred to hot dogs as Hot Diggity Dogs, thanks to Micky Mouse Clubhouse.  I miss that.  If she wants one now she asks for a hotdog.  I miss having to read the same Mickey Mouse book every night.  She has grown past Mickey Mouse now, gone are the days of Blues Clues, and never a thought is given to My Friends Tigger and Pooh.
I guess I am writing to help work through this sadness.  My husband just says "She's growing up" and that isn't much help.  I regret the pushing on towards the next developmental milestone and not taking the time to really notice all the amazing things she said or loved to spend time with. 
I remember when she was 2 being able to find the moon anytime, day or night.  I don't know where her fascination with the moon came from but she had it.  I remember one night getting her out of the car and starting to carry her inside when she got really excited and pointed at the sky and said "there's my moon!"  Even now, even though her little sister Ava has since gotten excited looking for and finding the moon, I still think of it as Katherine's moon. 
Speaking of Ava, I will never forget the night I was putting her into a pair of sleeper pajamas that someone gave us.  They were purple, had kittens on them, and had ruffles around the sleeves.  As I got them on her arms she looked at the ruffles and then at me.  She said "I a princess"  to which I replied "Oh you're a princess?"  She said "yes, Princess of world". 
These are my little girls.  Two very different people yet very much alike.  They are growing up eve though they are still young.  I know that the time will come when the stop making these comments full of innocence and humor.  I just hope that they still feel close enough to share their thoughts as teens and young adults, into adulthood.