Aww, I just love Fantasmic!

I have a lot of experience with both the dining package and "winging it", and both are fine. The reserved seating for the dining package used to be far right and not what I would consider primo seats, so for the longest time we just went without the package and got there early to sit in our preferred center section. I will admit that there is not a bad seat in the house because of the way the show and theater are designed. DD and I are just THAT into our Fantasmic! viewing. Now that package seating IS in arguably the best section, I booked it for our last trip. Sadly our flight was delayed by hours and hours, we missed the dinner, and got to the park with 10 minutes to spare before the show started!! This was on a crowded evening during President's Week, and as we were approaching the amphitheater, there was a voice on the loudspeaker exclaiming that the show was standing room only.

Well, we persevered and got up there to find that there were still seats available on the far sides of the auditorium, and they were perfectly fine. The show was wonderful as always!
45 minutes ahead should suffice, dining package gets you very marginally better seats, and you really can't go wrong! Enjoy!!
*ETA: Just noticed the age of your kiddos. Don't knock yourself out over the whole thing, because it may or may not even work out. The show can be scary for littles, and I usually notice at least a couple young toddlers being taken out. Don't let that deter you, but just be prepared to change gears if need be!