Remus went through a similar tizzy. I accedentally bought the Non-kitten Nutro once, I noticed my error and went back and got the kitten chow, hoping he would go right back to it...

Nope, my stubborn butt wanted the regular stuff! Sigh, he is gaining weight and developing ok so Im not too woried. So long as they don't develop problems I wouldn't worry. Maybe like Kim said, mix some of the kitten food in just to be sure?