my wife and i will be celebrating 29 years of marriage next week and we have 5 adult children, so i have a bit of experience in this area...

i have a few comments.
1. it is perfectly acceptable for you to express your wishes/desires/needs. doesn't mean you always get what you want, but it is fine to express that you would like to go out on a date with her. it is certainly a reasonable request.
2. any healthy relationship involves compromise. my wife is much more social than i am. i'm the one who is content to stay home every night. but once in awhile, we will go out with friends to meet her need/desire to socialize. i think it is reasonable for your wife to do the same for you. that might be once a month, or once every other month or whatever you agree on.
3. what would happen if you arranged for a sitter and then just told her "baby, put on your sexiest dress. i've got a sitter lined up and i'm taking you out tonight!"?
4. it is VERY important for a husband and wife to maintain their relationship AS husband and wife after kids start to arrive. my wife and i made it a priority to get away alone for an overnight at least once or twice a year - sometimes more often. i believe that decision is paying dividends now that the kids are (almost) gone.
good luck.