Mike's granddaughter seems to be younger in Breaking Bad than she is in Better Call Saul. Is this just one of those things that gets changed in order to make the story line work?
I believe it was an issue with casting. In the the Breaking Bad Season 5 episode "Madrigal" Hank questions Mike about an offshore account in Kaylee's name and says he has trouble believing that a fifth grader works for Gus Fring and that the money must be Mike's. Since this was around the time of Walt's 51st birthday, this should be 2009.
In the season one episode "Uno" of Better Call Saul, there is a scene where Jimmy holds up a paycheck with a date of May 2002 on it. Since the average age of a fifth-grader in the US is 10-11, then Kaylee should be around 4-5 in Better Call Saul.
I believe that they just cast the best actress that auditioned and she just happened to look "too old" for her character.