If you only do one? With a 2 year old? ELP, hands down, absolutely. No question in my mind whatsoever.
I just saw it on Friday.

First time seeing it since I was a child. Let's throw another one out...
I followed opinions found here (I think dangermouse helped with the place to watch it from). Watched it from almost where it starts, near Jumping Jellyfish. Seems counterintuitive, doesn't it?

So you watch it all from there, then it's done for you while it's still on its way to tSunshine Plaza.
Once it's done, walk purposely on the pathways towards Golden Dreams, turn left towards Grizzly, turn left to go by Grizz with GCH on your left (do NOT turn right at the waterwheel or you'll just be shunted back over to Grizz, LOL...saw so many making that mistake!), and then make your way past Soarin', and you'll be out the gates of DCA before Sunshine Plaza has seen most of the show. Depending on the crowds also doing that, of course.
Did I do htat part? No, I dawdled and walked in the street behind the parade until I turned left at the theater place. So I was slower, and the parade was about 3/4 done at Sunshine Plaza as I was leaving.
I didn't get a prime spot for the fireworks, but that's only b/c I met a "celebrity" cast member and he did his famed routines for me and we chatted for about 20 minutes, so I just watched the fireworks from behind a tree (which in winter was bare so it was fine).
I do want to throw somethin' out. Don't put your kid on your shoulders for a parade or the fireworks. Please? Just hold him on your hip and he'll see about what you can see, and you won't be making those behind you have murderous thoughts towards you. I have a 3 year old and I won't do the shoulder thing...he sits on my hip or in the Ergo on my back, and sees just what we see. And we're not blocking anyone else's view. Please.
I will also say that seeing the fireworks from back at IASW is a wonderful experience. And they let you sit down, instead of standing like you have to do near the castle.