Take a field trip to a farm.
Stage a very anatomically specific puppet show.
Tell them that new babies arrive when another baby gets wet after midnight.
I don't know, drop them off at the Family section of Barnes & Noble or something?
Have them ask their Dad.
Watch a horrifying 1960s filmstrip in 7th grade health class like the rest of us.
Tell them the hospital has one of those claw games, and it's full of babies.
Show them the video of their own births.
Tell them you have absolutely no idea.