oh wow. i have been looking for the exact same fanfic for YEARS.
i only read it once, but it was so good that i still remember it - and tried to find it again countless times, but to no avail. Yachiru was sent to the real world to grow up faster, and she came back as a young woman - but when she tried to do the same things she used to do as a kid, Ken-chan scolded her, saying "you shouldn't act that way around me. you're not a kid anymore." I want to read it again..
it would be really nice if we get to find that fic again..