I'd have no hesitation going with an 8mo, especially with two adults and one child (you outnumber him). He can go on any ride without a height restriction - which is most everything but the roller coasters and other thrill rides. It will be VERY hot and VERY crowded, so plan on standing in some VERY long lines. And I'm sure that the streets will be mobbed for fireworks - you may have to stake out a spot hours ahead of time.
You mention two rides at the MK, but I'm not sure that they have the best fireworks show of the parks. And MK tends to be the most crowded of the parks on weekends and holidays. Since the fireworks are available year-round, I'd suggest that you pick a different time to go when you can enjoy the parks more without the heat and crowds.
Single mom to
DS ("twice exceptional") - September 2002
DS - February 2006
DD - July 2009
DD - July 2009