The sentence in question:
その反面{はんめん}、あっという間{ま}に成長{せいちょう}しちゃうなぁと、少{すこ}し寂{さび}しい気持{きも}ちにもなった。
For full context: https://www.satorireader.com/articles/sanzu-no-kawa-episode-2-edition-m
My attempt at translation:
On the other hand, (I thought that) they grow to adulthood in the blink of an eye (and) I became a bit sad.
Well, I think that 思う or 思って (to chain the 少{すこ}し寂{さび}しい気持{きも}ちにもなった phrase to the ...と思う phrase) was just omitted after the と particle here, but I want to be sure that I'm right and didn't misinterpret the sentence out of flippancy.