We've come a long way from the days when adults would have to confront their sobbing parents with, "Is it true? Am I adopted?" If you're wondering when the book industry would catch up to the myriad possible answers to a kid's innocent question, wait no more.
The X, Y, and Me Before You Were Born... story books were created by Janice Grimes, a nurse in an IVF clinic--and an IVF parent herself--to "empower parents who wish to disclose sensitive information about genetic origin to their child from the beginning." [Or from when the kid asks; they're designed for ages 3-5.]
Each conception scenario gets its own book: IVF; Frozen Embryo; Donor Sperm, Egg, or Embryo; Gestational Carrier; Surrogacy, and Female or Male Partners. The only scenario I don't see covered is little Jor-El's arrival in Smallville via a bassinet/meteor.
X, Y, and Me books are available online for $14.97 each. If you need more than one, you should probably be on the Today show.