Like another poster, I never understand it when fellow DISers do not enjoy Fantasmic!. I think it is one of the most awesome things EVER, and I don't forsee a time when it will not be a must-do for our family on every single trip. We got in line absurdly early during President's Week (probably earlier than necessary), and even after the wait (which we really didn't mind, sitting in the amphitheater eating popcorn and playing with light-ups), we came out of it feeling like that was worth every minute of waiting! Both DD and I were like, "Was that not AWESOME??" all the way back to our resort. This was our 2nd time seeing it. Obviously I voted "yes".
I cast my vote before I read your post, though, and I have a very sketchy memory about how hard a toddler can be (and I never took DD to Disney at that age)!!! Only you can know how well it will go. The nappy-nap is a good idea, but you can play it by ear, or just plan on seeing it on a future trip (DO plan on it!). Even if you did the package, you still wouldn't need to stay around if things aren't looking promising. And we don't feel the need to do the package, if you'd rather eat someplace else.
Best of luck in your decision, and have a great trip!