In general, for pre school kids (Below 6), try to keep the character simple. Big facial expression (Big Eyes, Big Smile etc) makes it easier for kids to read what is happening. Keep camera movement to minimum. Less fast actions. Younger kids has less attention span. So normally one episode is around 11 mins.
For older kids (6 to 12), characters and story can be more complex. You can have more actions and camera movements. You also have longer time to tell the story. Around 22 mins.
Additional reading materials:
Character Study – The Appeal of Overwatch’s Characters