I strongly suspect that by the time teaching them about the "birds bee's" is an issue, the kid will have already figured out what gender they are on their own anyway. I figure this because a lot of trans people already knew their gender by a young age, and since nobody is raised to be trans, gender identity is probably pretty hard-wired no matter what you do. I mean, if trans kids can figure it out while being raised and told that they're the OPPOSITE gender from what they are all their lives...well then I can't think that simply not being told one way or another would cause much more confusion.

Maybe the kid will be more open and flexible about it for a while than a cis-kid who's told their gender from day one, but I strongly suspect that at some tender age they'll say "Mommy, daddy, I'm a ____" and that'll be pretty much the end of it.