okay, i can try to explain, but i'm generally bad at being concise... so i hope this doesn't confuse you even more.
what your Use Year means:
if you have a Dec UY, then every year on 12/1 you will be allocated a new allotment of points.
also, if you have a Dec UY, you will have the following points banking schedule:
you can bank 100% of your points before the end of May.
you can bank 50% of your points before the end of Aug.
you can bank 25% of your points before the end of Sep.
(btw, do you already have an understanding of banking points?)
common misconception: many people think that Use Year affects when you can make reservations. this is NOT true.
if you want to book a ressie for Dec 2004, you can call MS to book it 11mos out (Jan 2004)
regardless of what Use Year you have.
Use Year does not really matter to most people. the only time Use Year makes a difference is when you need to cancel a trip last minute.
for ex. if you booked a trip for Thanksgiving 2004, you would be using your Dec 2003 allotment of points.
(remember to think of your Use Years in terms of 12/1/2003 - 11/30/2004, and then 12/1/2004 - 11/30/2005, and so on)
in Oct 2004, you find out that for some reason you need to cancel your trip.
the points used for your ressie go back into your account... BUT they are 2003 UY, so those points expire on 11/30/2004. you have already missed ALL your banking windows by Oct, so that means you cannot bank those points into the next year to use. this pretty much means you'll probably lose those points.
however, if you booked a trip for May 2004 when SSR opens, and in Apr 04 you find out you have to cancel last minute, your points go back into your account. and you can still bank those points.
hopefully this helps you understand a bit more about Use Year?
in any case, for your family, if you really know for sure that you will
always vacation in Nov and May for the next 50 years (it's tough to say, right?

), AND you are worried about the risks of cancelling trips last-minute, then i would also recommend you consider an Oct UY.
the problem with Oct UY though, is that you will not get your first points until Oct 2004. so if you wanted to plan a vacation to WDW anytime before Oct 2004, you would have to borrow points.
one more piece of advice: generally it's too risky to travel during the last 2 months of your UY. i.e. for Dec, it's too risky to travel in Oct and Nov.
so, if you can more easily predict which 2 months of the year you are LEAST likely to travel, maybe you could try picking an appropriate Use Year that way.