Here's a take based on highschool biology courses I've taken long ago, but if anyone knows for sure, feel free to correct:

By amputating the distal part of the femur that has the cartilaginous tissues, you remove the part from which bone lengthens during childhood/puberty. 

For children, it means that the re-attachment procedure has to take into account the future growth or else, the knee-joint wouldn't be at the right level as an adult. Pretty sure they can approximate the position quite well with growth charts/family height history or something.

In case of an adult, you'd amputate so that the ankle is positioned at knee-height post-operatively since the bone stopped growing already = shorter.