TA, I am a second generation born in and it is my opinion that you dont need to completely shelter them from meetings or even relatives that are dubs. Like deus said above, encourage outside interests as much as possible.
If you remember back to your childhood years, like me they were not horrible but they were certainly "inside the bubble". When children only know life in the bubble they tend to stay in the bubble. BREAK THE BUBBLE!!
I have found with my two pre-teens that just getting them involved with sports, karate, dance, scouts, birthdays and sleepovers with school friends is all my kids have needed to simply live a normal life and view the dub lifestyle as.... lacking. Think of how awesome a gift you are giving your kids.... the gift of freedom that you did not have, you are breaking the bubble.
At some point you are going to have to take a stand especially with birthdays and other christian/secular holidays. If you are like me it just gets tiring after a while and you just say screw it, im out. Always remember this though, that YOU are not the one seperating your children from anything. Those "friends" and "family" that would seperate themselves from you and your children are at the end of the day, not worth keeping. Believe me, you will not lose the truly important family members, the ones who will not get in the way of you raising strong young adults.
Good luck, its a tough situation.
pbrow