I suppose I'm biased because I travel in party season and don't do the parties. We wear ourselves out just doing all the included things, that I feel comfortable not spending the extra for a party. Then I see reviews that sometimes it's pretty crowded or lines aren't all that low, or people feel like they need to do 2 parties to do all that's offered (trick or treat, characters, rides, parade, fireworks) and I feel pretty good about just doing more days in the park instead.
Add on that you're not sure your child will stay awake. Unless you and DH particularly love meeting characters (with or without an awake child in the pics!) and the fireworks enough to justify it, I'd go extra park days for sure.