Initial reaction: Freak the fuck out, probably be committed before school even starts, do months of intense therapy wherein I slowly realize the truth and bullshit my way through the sessions until declared sane. Then lament the next 15 years of dealing with Elementary School, High School, Hormonal imbalances brought on by puberty, inconvenience of growing pains and the meanness of kids to those who are different.
Think about it, if you're allowed to mentally mature (in my case) 26 years of life experience then find its all a lie, you won't be a kid anymore. Your innocence will be ripped from you and you will spend the intervening years between that moment and true adulthood pissed as hell. Plus most likely be ostracized because you're definitely "different" than the other kids no matter how hard you try to be "normal".
Plus those memories will fuck with you the rest of your life.
If you don't kill yourself somewhere down the line that is... which is a high possibility due to the trauma of temporal proportions.
I would not want to wake up if this is all a "simulation" or "dream" because I really don't want to have to "grow up" again.