I don't think it would be worth going to twice, IMHO. I'm glad I went but thought it wasn't quite as nice as I expected it would be. Do they both want to go? Or do you just think that one will be offended if you go with the other? Maybe Mom could go with the kids as a special treat and you could go with S-mom? (Or vice-versa, depending on who the kids get to see more of.) The bad part of that plan though is that you don't get to see the kids reactions.
You could do MVMCP on one night and the Candlelight Processional dinner program at Epcot for the other event. If CP would be too boring for the kids (it's lovely, but not many children will appreciate it) I think the Osbourne lights at MGM are the best holiday "event" at WDW. I like to get to NY St. early, get the holographic glasses and watch them throw the switch to turn the lights on. Then just enjoy the snowfall on NY Street and let the big crowds rush in, take some time to look around or have a snack and then go in when the crowds have thinned out. It was amazing to see all those lights!
If you do MVMCP, don't miss out on the riverboat cruise. They had Victorian carolers and the Country Bears roaming the boat, you can see all the lights reflecting in the water and everyone sings along. Just beautiful.