Funny, I sort of think of wDW as a no-brainer vacation. I know almost exactly what it will cost, I know where I like to dine, know my way around, know the location of nearly all bathrooms....
I'd totally pick October. I do enjoy the weather in April, but the crowds and pricing are crazy. (Though with the economy, they may offer some deals). DO read up about Disney 'season' pricing. (Try Mousesavers or teh parent of this site). This being a recession, crowds at WDW mightly be slightly different - but don't count on it. (esp 10/2010)
Deals for Oct 2009 aren't really out now, but they will probably offer something. The only deal now is the beginning of October - free dining, and that's probably pretty well booked up by now. REally it looks pretty good for deals in 10/09, who kows what 10/10 will be like. The economy may still be in recovery, or it could pick up by then...
Ages 4 and 6 is PERFECT for WDW! You WILL want to geta double stroller though. I think a Maclaren is perfect: lightweight, easy to fold, etc. The six year old probably doesn't use one much at home, but at WDW it will be very helpful. Unless you have a six year old who absolutely refuses to ride one.
I think 5- 6 days is a good time frame. Four is really tight, nine days of lines and crowds is about my limit. Current Disney pricing is geared towrds encouraging guests to stay a week.
Again, who knows what will happen in 10/2010. Disney could do away with Fantasmic altogether by then - or dramatically change it. I doubt they will eliminate all the parades and fireworks (they are so handy for getting folks off rides at the end of the night) - but the number of offerings could go down. They probably won't reduce shows in APril - they tend to do extra hours and shows then because crowds get SO crazy. Indeed, even this April - some days they reached park capacity!
October doesn't get THAT crowded, but it is still reasonable populated.
Both times of year have very good weather. October is still slightly hurricane season, but mostly they happen in August/September. (I think)
Currently Fantasmic is being offered twice a week - Mondays and Wednesday, I think. Not Weekends and not EMH days. So you kind of have to take your pick bewteen EMH or Fantasmic.
One thing I would encourage you to consider is staying onsight. The values are fine, the mods are nice, the deluxes are usually crazy expensive - but almost worth it. I've stayed at all three levels, and if there are no discounts: I'd rather stay at a value than not go. I love the moderate resorts, but I haven't stayed at one in afew years because the transportaion isn't the best. Before I go, I always think they are WAY too much money, but once Im there I think, "This was SO worht whatever we paid!"
The only offsite place I'd consider is the Swan or Dolphin. Both are very nice, I've stayed at teh Swan many times. They are right in the middle of WDW! Seriously! The only resaon I haven't stayed in a few years is that you can't use Magic Express or the dining plan there.