Your spur of the moment trip will cost $1800. You have $600 saved. Your bonus which is not even coming until you are in Disney will give you $500-$1000 to put toward the trip. Where is the rest of the $1800 coming from?
You still need $300-$700?
Just my opinion-I wouldn't spend $1800 on a vacation for just me and my child when I just went to WDW last year and I bought a new house.
Why do you have to give your FIL a "big chunk" of your bonus-did you need to borrow money from him for some reason?
Good luck with whatever you decide!