I would first check to see if all the seams where intact, no holes. After I eliminated any that had holes I would check for who had the best intersection matches. I would check for adequate quilting to hold the 3 layers together. Last I would judge on eye appeal and binding.
You could judge on colours and eye appeal only but that won't help them improve their construction on their next quilt. Ask questions from the 4H instructor on what they are looking for in the judging.