Here are some ideas to get the conversation started from Scholastic.com.
Tell me about the best part of your day.
What was the hardest thing you had to do today?
Did any of your classmates do anything funny?
Tell me about what you read in class.
Who did you play with today? What did you play?
Do you think math [or any subject] is too easy or too hard?
What's the biggest difference between this year and last year?
What rules are different at school than our rules at home? Do you think they're fair?
Who did you sit with at lunch?
Can you show me something you learned (or did) today?