Growing up too fast

I'm sure all parents feel this way, but as my oldest approaches 5 and the beginning of kindergarten, I am again reminded how quickly my children are growing up. Heck, the baby will be 2 months old next week. One of the girls' favorite movies right now is Mamma Mia (thankfully they're still too young to understand that plot of the movie, they just like the music). And every time I watch it or listen to the cd with them, I end up crying during the scene/song "Slipping Through My Fingers". How is it they're still so little and I'm already feeling this way. For those who don't know the song the words are the following:

Schoolbag in hand, she leaves home in the early morning
Waving goodbye with an absent-minded smile
I watch her go with a surge of that well-known sadness
And I have to sit down for awhile
The feeling that I'm losing her forever
And without really entering her world
I'm glad whenever I can share her laughter
That funny little girl
Slipping through my fingers all the time
I try to capture every minute, The feeling it
Slipping through my fingers all the time
Do I really see what's in her mind
Each time I think I'm close to knowing
She keeps on growing
Slipping through my fingers all the time
Sleep in our eyes, Her and me at the breakfast table
Barely awake, I let precious time go by
Then when she's gone, there's that old melancholy feeing
And a sense of guilt, I can't deny
What happened to those wonderful adventures
The places I had planned for us to go
Well, some of that we did but most we didn't
And why I just don't know
Slipping through my fingers all the time...
Sometimes I wish that I could freeze the picture
And save it from the funny tricks of time
Slipping through my fingers
Schoolbag in hand, she leaves home in the early morning
Waving goodbye with an absent-minded smile...
-Benny Anderson & Bjorn Ulvaeus

So I guess what I've taken from this is to enjoy each day with them as much as I can because before I know it they'll be out in the world living their own lives. They're already beginning to need me less and less each day. And while I'm proud of who they're becoming, I can't help but a little bit broken-hearted over it.