When I ask "How was today?" after we get home, my child says "I don't know." Is there a way to make it easier to talk about preschool?

Questions that ask them to freely look back over the whole day are especially hard when they're tired. Avoid right after coming home, and while looking at preschool photos or the schedule, ask specifically, like "Did you play outside or in the room?" or "Was lunch curry?" Don't turn it into a test of their answers; share one of your own events first, and if your child joins in with a word or an expression, that's plenty. For information you need, also use adult-to-adult methods like the communication notebook.