i have a 5 year old boy, who can be an absolute delight, but other times he is out of control, he has started prep this year, and i understand that he is getting tired and cranky, but he is acting out-

talking back to us
stirring his younger brother
crying if he doesnt get his own way
having real meltdowns and getting teary
wrestling his brother all the time
not listening to us

we have started taking things of him until he behaves.. i think a lot of it is when he is tired etc and bored. when he is on his own and its one on one, he is great.. also do you find that your kids play up more if you go somewhere like grandparents homes and they think that because they are away from home, they get all silly. its like each time i take them to my inlaws for a visit, they are so full on, and we leave and my mother in law is exhausted by their behaviour..
my 2 boys are a handful, and i am hoping that they slow down a bit. also how much concentration does a 5 year old have, its like he cannot sit still for 5 mins..
on a brighter note though, wow he has improved hand writing , reading etc since going to school , it often blows me away how much he has come in 3mnths..

i am just hoping that the full on, cheekiness etc is a stage as i am worried he is getitng out of control