system
nostalgic: My youngest child started high school this year. It made me think back to when she was just started kindergarten, and how time flies!
gpt
My youngest child started high school this year. It really made me think back to her starting kindergarten, and how time really goes by. I can just see her learning to read!
human
It is so hard and so rewarding to see them grow up. 
gpt
Yes, it really is bittersweet. But, the older they get, the easier it is to think back to the younger days!
human
I bet.  I really want to hold on to mine forever, but I know that they need to grow up.  I hope you are still able to make memories with yours even as they are getting older.