I went recently with my nephews - who were 19 months and almost 4 at the time. I really wanted to see World of Color, their parents really wanted to see Fantasmic, and I wanted to see the boys see the fireworks. We ended up seeing the three shows in 2 nights.
One night, we did World of Color. We got the picnic option, picked it up and brought to to our room to munch on before/after late afternoon break. Ended up breaking a little longer than Aunt Cathy wanted, so ended up in the top (back) section of WOC. There was a bench there, and nobody was using it, so I sat down, knowing that if someone came who 'needed' it, I'd have to stand. So I was in the front of the 'section' - but everyone else stayed near the back rope with the stroller, where they had a little more room. I have no clue if the boys ended up engaged in the show at all. I did end up seeing it a later evening (after they'd left) near Toy Story and I could see that it would be enhanced by actually seeing the surface of the water for the show. So for best viewing, I don't recommend showing up right before the show starts.
Our first night there we hadn't planned on seeing any evening show (didn't know how tired people would be/if they'd nap/etc) but ended up seeing the fireworks near the Village Haus restaurant, where there is no sound piped in. This was actually kind of nice, that way we could judge how the boys would do with loud noises and easily escape if it was an issue. No need for escape, whatsoever. The 19 month old also would clap in glee when particularly 'good' sets of shells were exploded. But that night didn't count! The almost 4 year old said they were better than Uncle T's (another BIL who lives where all fireworks are legal and likes to have safe fun on the 4th of July). (Shhhh don't tell Uncle T)
Because we have some crowd-adverse people in the group (including me) AND because one sister and I had done the dessert seating with our Mom about a year and a half before, we thought that would be the best way to be guaranteed of seeing it. Turns out my BIL is crowd-adverse, too and so we probably wouldn't have dealt with the crowds in the cold February evening to see the show if we hadn't had seating. Then, fireworks were before Fantasmic - so we went to the seating area as soon as we could so we would be 'settled' before fireworks started, and watched the fireworks (this time with sound) from there. Then watched Fantasmic. The 19 month old was free (as would your under 3), though he didn't get his own dessert or chair. They did have a kids price, too, for our almost 4 year old. Not having a seat for the 19 month old wasn't an issue, though, since he could see better on his other Aunt's lap - and we just kept handing him different goodies, and he was in hog heaven (the kid has only met one food he didn't like, and that was mustard greens!) The 4 year old wanted to leave near the end of the villians, because they were just too much for him - but we talked him into staying. The 19 month old again clapped in glee when "Mickey won" at the end.
SO, if you're determined to see the 3 shows with a young one, I'd totally do the dessert seating for Fantasmic and knock out both DL events from the seating area. But again, the villians are pretty intense, and the fireworks are pretty loud. If you don't know how they'll react, that's a lot of money to put down to have to leave because someone wasn't doing well.
When we went with Mom, fireworks were after the first Fantasmic, and we were able to stay in our seats to watch it.