Nope, not going to guilt you. Use your 2 year old as an excuse. It will be noisy, crowded (I'm guessing), hot, and late. Do you think she'll have much fun like that? Or even your 4.5 year old, who may just fall asleep?
I'm all for celebrating the 4th - I think it should be a priority for everyone to take a day to appreciate this amazing nation - but you can find other ways. How about doing some July 4th crafts together? Making flags from construction paper? Making a fruit salad of strawberries and blueberries, with whipped cream on top (keepin' the red, white, and blue theme going)? Watching "A Capitol Fourth" on PBS (fireworks by TV - not nearly as good, but can be an ok substitute in a pinch). Doing a patriotic-song sing-along. Take pictures of the girls making the flags, and use that for the scrapbook.