I haven't done the one here at DCA but I have done
MNSSHP at WDW twice. They are both fun, the one at DLR is just not as grand and therefore not as expensive. This year was the first time DL had Halloween fireworks,hopefully they'll be back next year. And unlike at WDW you can see them for free (well, inc. in the price of regular admission).
Also the hours for Mickey's Trick or Treat party are shorter than WDW. At DCA this year it was only 4 hours. (as cheap as $26 if you prebuy with AP) and at WDW if you arrive at 4 p.m. (like you can) and stay til midnight you'll get 8 hours. For which you'll pay around $55 advance. (These are 2009 prices).
Your nephews would enjoy either.
2009 dates at DCA were:
October 2, 8, 15, 21, 22, 28, 29 - 6:30-10:30 p.m.
October 9, 15, 23, 30, 31 - 7:30-11:30 p.m.