I am 29 and never had CP.
Our son almost 7yrs has never had it even though it went through the school like the plague and LO, nearly 10mths has avoided it too.
I was very worried when I was pregnant last spring because it was rife at his school and the MW's were concerned if I caught it and my GP was angry (!!!!) that I hadn't considered a vaccination BEFORE I fell pregnant (I didn't know there was one in this country).
In children CP is quite mild - it looks worse than it really is and although uncomfortable and a few sleepless nights it is classed as one of the more normal childhood illnesses. It is worse in adults that haven't had it and I def don't want to catch it now.
That said, I wouldn't vaccinate my two against it. I am not anti vaccination and I agree that the usual jabs must be done but for something which is deemed quite mild I wouldn't be putting a needle in them.
As smelly07 says if LO spends a lot of time around someone with a weakened immune system it might be worth a chat with the GP x