The important thing is that each course feels complete. Generally longer courses do well, but this isn't exclusively the case and parents may well feel that they don't have time for a longer course and so would prefer shorter courses. If I had that number of subscribers etc I would probably ask them a poll of what they would like me to teach first and offer a selection of different course options and use that to inform your decision.
I would then market that course (and subsequent courses) to your current audience to kick-start Udemy organic sales by demonstrating that the courses are popular.
My courses are all a mix of both long and short courses, but the length is never the consideration I've used, it has been about creating what I feel is a complete course, so if it take 10 hours to adequately cover a topic then that is how long the course will be, if it takes 2 hours, then it will be a two hour course.
All the best