Do you have an open or semi-open relationship with the parents of the child you placed? (If you don't, then I would hesitate telling your children at a young age all about him, because they may not understand why they don't get to know who he is. I might wait until they are older.) If you DO have contact, even if it's just pictures and updates, then it becomes so much easier to share it with your new in-laws. I have a friend who gets just a Christmas card every year with a picture in it. She posts the picture on her fridge, with other pictures of friends and family. Then when a sister-in-law came over and was perusing the pictures and asking who's this, and who's this? she casually said, that's the baby girl I placed for adoption before I was married. Well, of course there was surprise, but she made it no big deal, like other posters said, and it became no big deal. It was natural part of her. (That's how I found out, too - by seeing the picture on the fridge.) Her in laws (all LDS) think no less of her. In fact, they respect her for the decision she made, and see how it has helped shape her into the beautiful woman she is.
Good luck!
