I think also you have to think about what a child will believe to be "normal"- whatever home life s/he grows up with. Kids think that whatever their home life is like, that's the way everyone's home life is.
It all seems to boil down to how much you want to disclose to your kid. Figure out what kind of moral system the child will be raised with- would any of your activities conflict with it? If the grandparents are taking your child to church every week and s/he's learning about moral standards that conflict with your way of life, then it could be a problem.
Personally, I'd want to tell my child, but I'd spend the first oh... six years of their life figuring out how. Surely you're not the only person in the world that has had this problem- I'd suggest googling the topic and seeing if there's any good advice on it. Also, check out the book section at JT's Stockroom:
www.stockroom.com
__________________
Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi