That really is a tough one. I don't really know what to say, and I of course don't know anything of your family ralations, but with all due respect I would have to choose to honor my wife's wishes. It's always tough on kids to deal with death, especially at that age when life impressions weigh heavily.
To put myself in her family's shoes, I wouldn't take it disrespectful if you didn't make it, especially for the kids' sake. But this comes from and outsider looking in. I hope this makes some sense.