Are there any alternatives to "It's amazing how fast kids grow up ."

 Here are some alternatives to "It's amazing how fast kids grow up," categorized by the emotion or nuance they convey:

Focusing on the speed/brevity of time:

• "Time flies when they're little."

• "It feels like just yesterday they were born."

• "They grow up in the blink of an eye."

• "The days are long, but the years are short." (A classic and very poignant phrase)

• "It's incredible how quickly the years go by."

• "Where did the time go?"

Emphasizing a sense of wonder or nostalgia:

• "My, haven't they grown!"

• "I can't believe how big they've gotten."

• "It's bittersweet watching them get so big."

• "They're not my little baby/child anymore."

• "It's a wonder to see them change so much."

More formal or reflective:

• "The passage of time is so evident in children."

• "One truly appreciates the fleeting nature of childhood."

• "Their growth is a powerful reminder of how quickly life progresses."

More casual or colloquial:

• "They're growing like weeds."

• "Look at how tall you've gotten!" (Directly to the child)

• "Wow, you've shot up!"

• "You've really sprouted!"

The best alternative depends on the specific context and your relationship with the person you are speaking to.