Zizzle is open to flash fiction for a young audience:
At Zizzle we look for stories that will surprise, move, and amuse both young and mature imaginative minds. We like fiction that dives deep into meaning and resists banality. We believe the moral, if any, of a great story should evolve naturally from within and not be imposed from without. We embrace new perspectives to the nuanced joys and tensions in children’s daily lives and imaginations. We are open to pieces that defy traditional storytelling. Tasteful humor and a strong voice always appeal to us.
They're paying 100 USD per story for flash fiction between 500 and 1,200 words. If you write for younger audiences, check it out.
Listing here: https://literarium.net/market/zizzle/zizzle