That decision is left up to the discretion of the staff onsite at the Junior Race.
Participants will have to register in their corresponding age category and distance, regardless of which distance they ultimately intend to race. Then, on race day the staff will make their final decision based on the difficulty of the course, and the amount of racers in each heat.
Generally, older children are allowed to run in younger-aged courses (e.g. A 9 year old can run 1KM heat). However, letting younger children run in older aged is definitely left up to the discretion of on-site staff (e.g. An 8 year old running 1.5KM heat).