I am writing children's books. One mantra I try to keep to is to make my chapters ~15 minutes long when read aloud by a parent at bed time. This gives them the ability to be consistent in their bedtime routine.

This way children don't get short-changed and adults don't find they have committed to something they can't get out of. It also allows them to easily give in and do another chapter — after all, it's only 15 minutes.

30 mins reading out loud is about 4,500 words, so 15 minute half-chapters are 2,250 words.

A 2,250 word chapter is also very manageable by a child.