I haven't posted in a while, but I had to put my two cents in.
I can't understand your reason for not making the guest list under 21. Whether or not it is about money, you have to be honest with yourself. Is it that you just can't stand giving your FILs their way. If it's about money then why are you against your FILs paying for the guests they want? If they want so badly to invite the x # of cousins under 21, so be it. How does it affect you negatively?
It seems that you just don't want any young people there. And if that's the reason then be honest and have the guts to defend what you feel. ie: I don't like kids, and therefore don't want them at my wedding. End of story.
Another option that's also been mentioned make it a 16+ guest list. Not enough guests excluded yet, 18+ and the three exceptions you have already made.
Finally, if this is your idea of pushy FILs you should thank your lucky stars. I think the biggest reason they are pushing on the guest list, is because they don't understand your reasoning. Enlighten them as to what your reasons are, and assuming they are legit, they probably won't have a problem.