Hum ... perhaps the daughter lived, emigrated to europe and thus is buried elsewhere. :-)
Don't think I'm seriously defending the theory, but the fact the daughter's body was not there doesn't run counter to the Da Vince theory. And the boy may not be mentioned because it died in infancy - lots of that going around at that time.
People who make claims like the ones in the Da Vinci Code based on his painting of the Last Supper make me laugh. First of all, everyone in that portrait...is Italian. That's because Da Vinci wasn't really there. What if he had put a monkey on a unicycle in it?