Fiancé visa expiration?

Hi! I will be traveling down to LA to obtain my fiancé visa mid March. Once I receive that, and go over to the UK, do I have 90 days to get married or 6 months? AND. . . my Children will be going to the UK with me as well. Am I correct in understanding that I must fill out the IM2A and IM2B for each of us and also pay the application fee for all three of us .  So, the fee today is about $429.00 for each passport – $1287.00 in all.  Am I correct in my understanding?  
Melissa

I'm afraid that's right Missy, you have to pay for each visa you apply for.  Sucks, but that's the way it is.  

You have 6 months from the day you receive the visa to get married.  So if you get the visa in mid March, you have until mid Sept to get married and apply for the FLR visa.  Even if you don't come to the UK until August, the visa will still expire in September.