biologically no, it would not affect thier bodies to much degree beyond aiding in a better physical condition. Children have a natural tendency for movement and a lot of it over the course of a day.
Pediatric study for children states that female children need at least 12,000 steps per day while male children need at least 15,000 steps per day to maintain an average BMI scale for a healthy child. An adult should be walking at least 10,000 steps a day. So an adult needs less steps than a child.
The above being true, the average marathon consists of 33,000 steps, if an adult needs much less steps to stay in the healthy zone then logically a marathon would be less healthy for adults who require less steps. But if we read carefully, children require at minimum 12,000 for girls, 15,000 for boys. So a marathon would be more than healthy for a child. It would allow them to develop faster and stronger than most children who are barely reaching the average.