I’ve been writing for children and teenagers and teaching creative writing for many years. I find that starting with setting and then discovering characters is the best way to get a story started.
Working outside or visiting an interesting place really helps. My students loved working that way, but once the research and the ‘creative daydreaming’ has been done, it’s vitally important to be disciplined enough to go back inside and start work.
The combination of exploration, free thinking and concentration is really effective. Try it.