One thing I was told a while back, that I have to keep reminding myself of, is that there is nothing in the parks that is so worthwhile that it's worth messing up your vacation. It's easy to read all the planning information and start thinking of things that must be done, and that will ruin your vacation if you can't do them. Trust me - there's nothing in that category.
With regards to fireworks specifically, they're great, but they are one of many great things at WDW. If it's going to throw off your kids' schedules significantly, and thereby cause problems for your family, then no, it's not worth it. We've been fortunate enough to go to WDW several times, and there are many things we have skipped, especially when kids were younger, that would have been nice to do, but we decided we couldn't. We've never regretted it in the long term. We had to get over the "pack as much as possible into a day" mode so that we could be in the "enjoy your entire day" mode.
Now, that said, your kids probably will end up with a different schedule on the trip than at home (it just happens, even when you try to keep a very regular schedule!). Others have given some good suggestions, about naps, and adjusing your overall schedule, etc. Another question is what you'll be doing the next day. If you have managed to build in a rest day following the MK, then it might not be so bad to keep them up late, and let them reset the next day. Also, if you have a stroller and your kids can sleep in it, then that's another option.
To your specific question, Robo answered it well - better fireworks views == longer crowds. And, at the end of the day, you will experience huge crowds on leaving - do not plan on being out of the park and back at your hotel in a short amount of time.