I was just telling my 13 year old son not to worry too much about a bit of fat. He's very active and tall for his age. I'm not a doctor but I think that some kids naturally show abs and some don't. And for the latter, reducing calories to the point where they would show abs might hinder their growth.
I was pretty chubby at age 12 but grew to 6'3", 190lbs by age 15 and was pretty lean (still no sixpack, though).
I think you're better focusing on a good diet and the type of functional exercise Crossfit offers. Excessive weightlifting is also not good at your age -- your growth plates need to close and your body needs to develop further, otherwise you risk permanent damage.