Did they know it was your anniversary?  Does your family celebrate anniversaries?  I've found most kids think their parents anniversary is more between their parents and not so much for their participation.  The fact that it wasn't acknowledged is not a big deal but it is something to remember if you're expecting a big party on your 50th.  Plan that one yourselves and invite whomever you'd like.  This isn't something to be hurt over or to hold a grudge over.  Your kids have their own lives and are probably very busy trying to stay ahead of everything.  Relax, take a deep breath and ask yourself, is this one of those battles I will choose to fight?